The £20 service is pointless - be it LBS or
Halfords! Well, it should be - you should be more than capable of doing what happens as this service yourself.
My LBS does a 20 'service'. They are very open that all they do for this is check the brakes, the gears/chain and the tyres. They fix/alter/lube/service nothing, they just simply check them for safety and let you know if there's anything dangerous. They also let you know if anything needs work and what the price would be for it.
Essentially what any cyclist should be able to do in five minutes themselves!
Check exactly what they sell and do in the service. I'm more than willing to pay my LBS for their 'Deluxe Service' where they do the lot, true wheels, strip and regressed bearings, replace cables etc etc but the cheap 20 quid one is to be avoided.