That's what I used to do for revalidations.
Until, that is, some nurse decided to use the sphygmomanometer just after I'd had the bloodletting.... The resulting bruise a day or so later looked as though I'd been bitten by a horse. A civvie quack just uses a small finger-pricker and a slide, then puts the slide into a machine for an instant read-out. Whereas most RAF quacks have no such machine, so it's an armful of blood off to the lab and wait...
The other problem with RAF quacks doing the Class 1 is that few hold the official stamp which allows them to issue the certificate - so you have to wait for your paperwork to go to Gatwick and back again. Allow for that if you need your certificate quickly.