What should I expect from a bicycle service?
These services vary wildly from shop to shop. 

My local shop will look at any bike and tell the owner if it's safe to ride. And of course, they'll tell the owner how much it would cost to make it safe if it's not already. They're pretty honest about it, but of course I can't guarantee that other places are.

Most shops offer a free "tune-up" with the purchase of a new bike. This usually includes minor maintenance and inspections such as tightening bolts, airing up tires, adjusting brake and derailleur cable tightness, checking that the wheels are true, etc., but no parts replacement. If the bike was not purchased from the shop in question, they will usually offer the same tune-up service for a relatively low fee, $25 (~£16) US where I live.

Many shops offer additional service levels beyond this. They may include replacing cables and/or cable housing, bar tape, brake pads, actually truing the wheels (this can be a more time consuming job and may not be included in a basic tune-up), and perhaps even a bike wash. These tune-ups can get fairly expensive, $100-$150 (£67-£100) US. 

Exactly how much you pay for any tune-up will depend on exactly what combination of services a shop offers and the amount of local competition, so you should really shop around.