what about renting an appartment or similar?
I'm going to France in july with 4 friends and my boyfriend, we're all aged between 17 and 19, and there are 2 couples going. We actually found it cheaper to book all our own flights and get an appartment for the week than to go in a hotel, it was easy to do. The only problem we had is that the person we're renting it from asked us for double the security deposit because we're under 21, my mum had to vouch for us and say we're responsible, mature and aren't going to cause trouble etc but that was it.
or, when contacting the hotels you don't have to tell them your age I don't think. After all, they wouldn't ask a 30 year old their age so unless you tell them how old you are I don't think they'd ask, as its rude. Have people asked you or did you just give your age? You're fine having rooms in the UK at 18, and younger than that, like Pumpkin7 said. I've stayed in hotels all over the country with my boyfriend for various things, the most we've had is a few "tut, tuts" from old people.
Travelling on your own or with friends is straight forward enough, make sure you know important things like addresses, flight times etc before you go, and you'll be fine. I usually end up being the 'leader' of the group as I'm the only one who is bothered to sort out the practicalities, as long as you know what you're doing and keep your wits about you its basically the same as being in the UK, you just have to do the thinking instead of having parents etc to think for you.