As someone else stated, is it a truly a 5 min job or more than that?
I remember on my first cars in the 90s, it was literally pop off the rubber cover on the back of the light, twist the bulb housing out, remove, replace, put back together. Two minutes max. Today with some cars you may have to go as far as to jack it up, take the wheel off, wheel liner out, remove bumper, remove headlight... you get the idea. Sadly one of mine is like this. Hours of work. It's an extreme example but want to make the point your car could be a little more complex than the way things used to be (5mins).
Anyway, £48 sounds like about half an hour of a garages standard labour time. Looks like they charged that, which may well be their minimum for all/any services. Other garages may do it for less or have a set fee for bulbs, others may do it for free since they have a relationship/ongoing business with you.
What I would do? Check to see if yours is indeed a 5min job. If it is, maybe ask them if they could refund it. Nothing to lose. If they stand their ground, politely inform them that's a shame and that you won't be returning to them.
Like most things in life, some businesses will let the little things slide for the sake of ongoing repeat business and reputation, others will want to nail you on every little thing. Choose who you want to do business with based on that.