Essentially there are a couple of extra steps now. This applies to fully vaccinated passengers.
It's probably a little early to say for sure how things will work for US flights but I would say things should be similar to other destinations.
1) Pre-departure testing (generally required no more than 3 days before departure)
Do check which type of test is acceptable as this does vary for different destinations.
For example Barbados will only accept a full PCR test with swabs from your mouth and nose administered by a professional.
USA will accept a PCR, antigen flow lateral or a LAMP test administered by a professional of performed yourself at home BUT home testing MUST be supervised remotely via a video link.
I'm doing a drive-thru flow lateral test for my US trip as I've read a few negative reviews about some of the supervised home testing companies keeping you waiting for hours in some cases for an available person to monitor you take the test even after making an appointment.
I've booked Express Test drive-thru for mine (£50). I also used them for my previous PCR test (£80) for Barbados simply because they where the closest to where I live. Very good all round service for the first test last month with PCR results back within 24 hours. They state flow lateral results are returned within 40 minutes. I'll find out for sure tomorrow when I go for my test ready for Wednesday.
Do make sure you enter your passport number when booking your test appointment as this is required for International travel. The test centres require you to take you passport with you as well for the pre-departure test.
There are discount codes knocking around for Express Test plus they are also on Top Cashback now as well (not sure about Quidco?)
Many other test providers are available though at various locations around the UK including all airports.
Again though; check which type of test is required for your destination.
2) UK Airport Check-in
I can only write what I experienced at MAN. Other airports may be slightly different.
An agent checks your pre-departure negative test certificate and vaccination status before letting you check-in properly.
Once they are happy they attach a red security sticker to your passport allowing you to check-in normally. That took no more than a few minutes at MAN but bear in mind that was 1 single daily VS flight. The rest of check-in exactly as normal apart from the agent asking you to briefly remove your mask to check your ID.
From Monday check-in is going to be busier so do allow extra time and have you test certificates and vaccination status ready for checking. I printed mine out.Do remember that the NHS app requires a data connection to work so if you don't plan on using data roaming outside the UK, you will need paper copies of your vaccination status to show once you leave the UK. This can be downloaded via the NHS app saved somewhere offline or printed out.
iOS 15.1 now allows your NHS vaccination status passes to be added to Apple Wallet which show passes without a data connection.
3) Passenger Locator Form
48 hours before arriving back into the UK (based on your flight arrival time into the UK so you need to take any time difference into account), you need to fill in your passenger locator form (PLF). This is an online form which can be found here
I found this to be very hit and miss and it took be 3 or 4 attempts to fill it in and submit it. Part of this requires you to scan your vaccination status QR codes. I found this to be a real pain as it's kept timing out on me. I managed it eventually.
Another section of the PLF requires the reference number for your return to UK day 2 COVID-19 test. This needs to be purchased before you fill the PLF form in and most companies will deliver it on or around the day you return home. Cheaper flow lateral tests can now be done of this test if you are fully vaccinated.
They issue you the reference number as soon as the purchase is complete so make a note of that and take it with you to enter into the PLF.
Again many companies sell these now. I'm using Chronomics which was £23.50 for a antigen flow lateral home test kit.
4) Departure Airport Check-In
Flying back from Barbados there was a pre-check in agent again and I was required to show my passenger locator form and vaccination status. (I was also asked to show my departure negative COVID result but this is no longer required now)
I saw passengers struggle here as they had no data connection to open their NHS app to show their vaccination status.
Nothing extra was required for arriving through UK immigration on landing at MAN. E-gates where in operation and working.Disclaimer
Do read and make sure you understand all the requirements for travel to and from your destination carefully though as any incorrect requirements may lead to being denied boarding. Check regularly for any updates as requirements do and are changing all the time.