I was 'obviously gay' at school although not obviously out although a few people knew and if someone asked directly I would tell them and I did get bullied I suppose you would call it although thankfully, I had a good circle of people in my life so it never really bothered me.
I have to say, most of the bullying I encountered came from the staff - I also am from England, but I left school 14 years ago (argh! I'm old) - because my first name rhymes with 'gay' it was an easy 'joke' for people to make and the staff NEVER did anything about it if they overheard it.

To be honest, I think a lot has changed, I know my ex who is a teacher tries to combat the 'thats so gay' comments and tries to educate and enlighten those that use it. Generally, most UK schools have an anti bullying policy which does also cover homophobic bullying too so, in theory, it shouldn't be as bad now as it was when I was at school. If it bothers you a lot, maybe try talking to a member of the staff and see if it can be addressed as it would count as offensive if not some form of bullying.

As far as 'thats so gay' goes, its not limited to schools, my brother still says it and he's 26. I usually reply with either; 'shouldn't these things come out in their own time', 'OMG that's so straight!' or something similar. I even went as far as putting the 'so gay' things (tv remotes, phones, whatever) into positions to make them look like they were well, performing adult acts shall we say and he's since stopped saying it.

To some extent, I think these things are only said to try and get a negative reaction so if you turn it around, then it loses its power. I don't always think that they are intended to be offensive, and yet, if you started saying; 'thats so straight!' people would suddenly realise how stupid and ignorant they sounded and stop.

Just to be clear, I'm not advocating any retaliation or anything of the sort.

I was never a problem for me in high school, I knew if i came out it wouldn't really matter to anyone and I wouldn't get bullied. In College though things changed. I made friends with people who didn't know I was a lesbian. So every know and then when they are making a point over someone they are having a problem with, they start using the words, gay, faggot, dyke, homo, I was kinda shocked at first and now I don't tolerate it. To replace stupid or idiot with any of those words is offending me, who I am in my entirety. They don't say it around me anymore and I hope they stopped completely. Little things like this is where it starts...