It is easy to get an Annual done for £300.
(1) Do the work yourself and value your time at zero value
(2) Do not use any significant parts
(3) Pay a mate of yours £300 to sign it off
Actually I know a bloke who used to get his piston twin annuals done for £300. He was a CAA LAME and did them himself, including the signoff. Oil (cheap oil) costs about £80 for a twin, two oil filters at £12 each... what else? All the rest is
LABOUR
That's why all these claims are thoroughly disingenuous.
A repair on my lawn mower, done by the shop, costs me £300.