You "only" need a lateral flow test to enter the UK, which you can do by videocall whilst abroad. Book a pack of tests before you go, if necessary one for when you enter the foreign country and another for the return. Breathe Assured seem to be amongst the cheapest providers for that; with an airline discount code it's £68 for two tests.Where have people here gone (or are planning to go) for their tests? I'm interested in tests before arriving in the UK as well as tests two days after arrival.
For the day 2 (PCR) test, I would just choose the cheapest provider you can find - the only vital bit is that you can prove a booking at the border. Randox seems to do the job at £43 with an airline discount code. There are reports of them being unreliable but this doesn't matter as there is no real penalty for failing to take the test if you are a "green list" arrival.
In all, the testing will set you back at least £80 per person or so even for a "green list" arrival, which is a complete ripoff, but at least it's no longer as expensive as it used to be.