Blood tests? Where? UK Based people

Hi,

Might seem a silly question but I went into the GP specifically to ask for a blood test, they refused without an actual reason for having one done.

Was a bit shy by giving them the real reason (I'm pre-cycle and I'd like to know my baseline etc). This is the first time i'll be doing blood testing before, mid and after (yes I know I should have done them my previous cycles too). If I say I'm on a steroid cycle do they out-right refuse?

What should I say to the GP for a blood test, or are there any particular sites I can go to? I saw www.thriva.co - blood tests are £99 and self administered.

Any help appreciated