The prices of flights and hotels for new green list travel destinations have swung sharply in both directions since the guidance was changed on Thursday.
The government added 16 destinations to the list. Travellers returning from these do not have to isolate on return.
Destinations include Malta and Spain's Balearic Islands, and some holidays have doubled in price, the Independent's travel editor Simon Calder said.
Airlines have added extra capacity.
Initially when these new flights have become available, prices have fallen as extra supply takes up the demand.
But this has proved to be short-lived in many cases: "Availability is changing by the minute," said Mr Calder, who has been checking prices regularly since the announcement.
"Immediately after it was made there was still quite a lot of keenly priced flights and holidays, but we're back down to 'last seat' showing."
"Madeira and Malta, for example, were looking pretty reasonable, but then things really took off. Unless you're sitting in front of a screen there's no way of gauging which trend line you're on," he added.
EasyJet said: "When new flights are put on sale the starting fares may be lower compared to existing flights with many seats already booked.
"Like all airlines, our pricing is dynamic which means that our fares start low and increase the closer it is to the date of departure and as more seats on the aircraft are booked."
The latest list changes:
The destinations added to the green list from 04:00 BST on 30 June are:
Europe: The Balearic Islands (which include Ibiza, Menorca, Majorca and Formentera), Malta and Madeira
Caribbean: Anguilla, Antigua and Barbuda, Barbados, British Virgin Islands, Cayman Islands, Dominica, Grenada, Montserrat and Turks and Caicos Islands
Other British Overseas Territories: Bermuda, British Antarctic Territory, British Indian Ocean Territory and Pitcairn
Six destinations will also be added to the government's red list on 30 June. They are the Dominican Republic, Eritrea, Haiti, Mongolia, Tunisia and Uganda.
Operators contacted by the BBC were concentrating mainly on satisfying demand for the Balearic Islands, Majorca, Menorca and Ibiza, and Malta.
BA said it was reviewing its schedule and had added some additional flights and upgraded some routes to larger aircraft, for example to Malta and the Balearic Islands.
EasyJet said it was also adding seats and holiday packages and new routes, also focussing on Malta and the Balearics.
New routes to Malta from Bristol and Luton will launch next month and offer some seats at £27.99.
Ryanair said it was selling 200,000 extra seats on flights from the UK to Malta, Ibiza and Palma for July, August and September.
Ryanair's Dara Brady called the latest announcement a "small step in the right direction" but called on the government to add Cyprus, the Canary Islands and Greek islands to the green list.
Jet2holidays said there had been a "huge surge" in bookings.
It reported the highest volume of bookings to the Balearic Islands in almost a year, with bookings for July alone up more than 3,000%.
Bookings for Malta and Madeira also jumped by almost 1,500%.
How do airlines set ticket prices?
Several factors influence the price of airline tickets. These include how many seats are available, how many people are looking for tickets, the popularity of the route, and the time of the day the journey takes place.
Due to computer algorithms, airlines can alter the price of tickets by the minute. For example, an algorithm can pick up when lots of customers are looking to purchase a particular ticket and then trigger an automatic response.
However, it's not all computers that set the price, humans play a role too. Revenue managers will sometimes manage automatic systems from time-to-time.
Peter Morris, chief economist at Ascend consultancy and a veteran with 25 years in aviation, says approaches do differ between airlines and they typically sell tranches of tickets with prices increasing as long as sales are going well.
Jet2.com and Jet2holidays have put more than 70 additional flights on sale to Malta and Madeira with immediate effect and a new route to Malta from London Stansted is being added.
Steve Heapy, chief executive of Jet2.com and Jet2holidays, said the response from customers had been "truly incredible".
"Bookings to Majorca, Menorca, Ibiza, Malta and Madeira have gone through the roof, which shows just how much UK holidaymakers want to get away."
The Independent's Mr Calder said: "It's now a matter of 'can you time it right' so you go in just when the new flight goes on.
"If you do you can still get a bargain. Otherwise you are competing with people prepared to spend many of hundreds of pounds to get away to what are, after all, still a very small number of destinations compared with normal years."
The new rules on travel will, of course, change as situations develop.
And the countries on the UK's Green List have their own set of rules.
For example, Malta will only allow people in to the country without quarantine if they present an approved valid vaccine certificate.
An email from its High Commission in London said on Friday afternoon: "This means that only people who have a vaccination certificate recognized by the Superintendent for Public Health can enter Malta from the United Kingdom without the need of quarantine."
It said anyone who did not meet the entry requirements should make a formal request to the Public Health Superintendence to authorise entry to Malta at email@example.com. Further conditions may then apply.
Transport Secretary Grant Shapps has said the UK government plans to drop quarantine for fully vaccinated people returning from amber list countries "later in the summer".
However, EasyJet has said the timetable "simply isn't ambitious enough".
The transport secretary said issues such as whether children should be given the vaccine and how people from outside the UK could prove their vaccination status needed to be resolved before the system could be introduced.
He added there was also a "fairness issue" with young people yet to receive their second jabs.