I am not against what everyone has had to say, in Joanne's situation I think it would be inappropriate and too early for your daughter to go on holiday with her boyfriend ALONE. My opinion is that the decision of whether you will allow your child to go on holiday when s/he is 15 years old or above shouldn't solely rest on an issue of age. This (in some cases) can be an acceptable age to travel alone or with peers (depending on the circurmstances of the holiday). It should be both the type of holiday as well as the age of the child that bare influence on any decision you may reach.
Now dealing with the matter at hand - Joanne. I think for the circumstance (that being going on holiday with her boyfriend) your daughter is too young.
I would say if you have any doubts at all about what your daughter may get up to then don't allow her to go. They will obviously share a bed and no doubt you wouldn't approve of this in your household with you there (as she's only 15), so why should you approve of it elsewhere when you're not there?
If you feel that when she becomes sixteen that will be an okay age (though I'm not too sure myself - it depends), then perhaps suggest to her that they go away in the Easter. Personally I think 17 is a more suitable age to allow your child to pack up for a holiday with their b/f or g/f.
If her boyfriend's parents were to accompany them as well then the situation would be fine.
My mother dearest (not a bad mother at all, may I add) allowed me to go on holiday when I was 15. I went to Berlin for a week with a group of 15 teenagers and returned the day before my first GCSE exam. I didn't get up to too much trouble!

The first night there I wanted to explore the city as it was so beautiful... totally forgot that trains don't run all night and realised that myself and one of the girls that had gone were still out past 1am. We got pulled over by police jabbering on in German. They finally spoke English and told us that children under 18 were not allowed to be out at that time without an adult! We told them that we were staying nearby, but truly our place was probably 7 train stations away. We also got entagled in the May Day riots as our hostel was a stone throw away from where the protests take place. We met lots of quirky people, saw lots of strange things.... but that's what make life worth living. Don't rob your child of experiences. If you know that aside from a little wildness your child will mostly behave in a respectable manner when away from home - then give him/her the opportunity to have fun. Condoms were thrown at me on the train ride there.... I was offered sex and drugs. I declined both. My mother let me go because she knew I wouldn't do anything worrying, I'd just have fun!
Ahhh... memories.
Choc Chip
P.S. Don't let her go alone with her boyfriend
"my classmates would copulate with anything that moved, but i never saw any reason to limit myself" - emo philips
"my girlfriend said to me in bed last night, 'you're a pervert'. i said 'that's a big word for a girl of nine'" - emo philips