My first piece of advice would be to get the test in advance, read the instructions fully (in my case several times!) and take your time.
This company, Eurofins, are by far the cheapest that I've found:
Their 'Test to Depart' is £44.90 and their 'Test to Release' is the same and they're government approved.
They have drop off points around the country or you could possibly take the test to the post office and have it upgraded to signed for or special delivery.
I used Medicspot when I flew (also government approved). Much more expensive (I didn't know about Eurofin!) at £149 but I went and collected my test, did it in the car and gave it back to them. They have couriers collecting every day (that the individual pharmacies are open) and seem to have a bigger network of places to collect from. They are offering 10% off testing with the code below but that's still considerably more than Eurofins. I suppose it'll depend on whether it's important to you to be able to drop the test off etc. (I get a £10 Amazon voucher, incidentally for each test booked with Medicspot, but would still suggest Eurofins - so much cheaper!).
: Whichever service you use, check the time that the courier is expected... The pharmacy I chose closed at 1pm and I got there at 12pm thinking I had plenty of time, only to hear that the courier was expected at 12.30pm! If I'd left it to the last minute I'd have been scuppered...
There are plenty of other companies out there offering test but Eurofins is the cheapest I've found and Medicspot are the only ones I've had direct experience of.