It is very unusual for a garage to do that. the usual drill is for you (or them) to say when booking it in "any minor repairs up to £50" (or whatever) "just get on and do them"
That saves you having to go back another day to fix a blown bulb or worn wiper.
It is normal for them to phone and ask what to do if there is a repair required above the agreed limit. For all they know you are handy with a spanner and can change a bald tyre, fit new shock absorbers or brake pads, or whatever.
Either they have made a mistake, or they are trying to take advantage.
If this is the garage you regularly use, phone the service manager and say you don't think they have acted correctly. Calmly and politely, of course. Then stop. Don't say any more and wait for him to reply.
If you are not satisfied, never go back.
BTW if they did the work withgout your instructions, you are only obliged to pay "a reasonable amount" so you need to know what it was. I hope they gave you an invoice with breakdown of parts and labour. if not they are on sticky ground and they know it. Don't take your car back until this is resolved as they are allowed to keep it untiul you have paid
and charge you storage fees and even sell it for scrap 