Is COVID testing to return to US from London a problem and pricey?

You're reading the rules wrong.

Your return test must be no more than one day prior to your flight. If you flight is on a Tuesday, your test can be anytime Monday and up to your departure time.

Tests are not hard to find and run about 30 GBP give or take a few pounds. You want an antigen or lateral flow test and not a pricier PCR test.

The UK has no official quarantine anymore. It is up to the hotel whether or not to take you but most will. As long as they have rooms available.

If your airfare is refundable, then you shouldn't have a problem. If that's a typo, then your NON-refundable airfare cannot be refunded but most airlines will allow a change at no charge. That's no charge for the change but you will have to pay any difference in airfares.

These are the reasons why we suggest getting travel insurance to cover these issues.

https://www.nhs.uk/conditions/coronavirus-covid-19/self-isolation-and-treatment/when-to-self-isolate-and-what-to-do/

You won't be stuck for weeks unless you get so severely sick that you can't fly.

And it's not 48 hours, at least for the US. It's one day. NOT 24 hours. NOT 48. One day - so if you fly at 5PM on Monday you can test anytime from first thing Sunday morning until shortly before your flight.

What could happen is that you test positive and need to stay until you can get a recovery letter. This is a statement from a doctor (there are online services to meet with the doctor, you don't need to have a UK doctor) that says you tested positive and are now safe to fly. This can be done 5-10 days after your positive test if you are no longer sick. Check with the airline, I've heard some are OK with 5, others with 10. Anyway, after 10 days you can go home.

There's an extremely annoying story in the NYTimes about a tourist who kept testing positive and thought she couldn't come home until she had a negative test even though she was recovered. FALSE, this tourist was uninformed and spreading the misinformation. You can return home with a recovery letter and a positive test that's at least 5-10 days old. Irrelevant to your situation but someone who was positive at home in the month before their trip could travel for a month and return with a recovery letter, no new test.

Frank’s answer is excellent.

It is true that the test to return adds another, very stressful, element to travel. It is not difficult to find testing centers in London. They are conveniently located near major transport centers.

There's an update in the recovery thread in the COVID forum that says QuickMD has switched to a 10 day period (i.e. not 5) to get a letter of recovery.

https://community.ricksteves.com/travel-forum/covid19/getting-documentation-of-recovery-if-you-test-positive-while-overseas

I made a mistake. I meant to say non-refundable airfare.

Also, I asked about prices for tests.d My kid had to pay 50 Euros in Italy an later, from Austria, only 7 Euros for the same test. Where are the cheap ones?

On 4/29 I paid £32 ($40 USD) for my fit-to-fly antigen test at Collinsons, upper level of St Pancras. In at 10:20, negative result via email by 11 am. As this was my last day in London I chose this company for its proximity to the British Library, and then a 15 minute walk to the British Museum.

Does anyone know if there is a travel insurance that specifically covers the cost for an extended hotel stay and flight fare change for all travelers in the party if one of them tests for COVID and can't take the original flight back? Perhaps I am posting this in the wrong place...sorry.

I think that you would need to contact your insurance company with that question. I would say that it is to your advantage to have as many as are able to return as scheduled to do so. Otherwise they might change from negative to positive and further prolong the delay.

I know in some situations it might be necessary for everyone to stay, and I hope you find the answer to your question.