The Office for National Statistics says 480,000 (1%) consider themselves gay or lesbian, and 245,000 (0.5%) bisexual.
Emphasis mine.
I'm less concerned than joyboytoy about not including youths in the survey - asking youths about their sex lives is asking for trouble with ethics boards, parents, and gonzo politicians. Besides, teenagers are still deep in the identity-formation period and will eventually become adults anyway. Not asking them is a problem, but it's one of pragmatics and I think you can generalise from the 16+ population.
My big issue is with asking for self-reports. This is a notoriously unreliable way to assess who is actually attracted to the same sex - many researchers have been trying to get away from self-report for decades now. Of course, this is because some people lie to fit who they think they are, some people are deep in denial, and others simply assume that they have to be gay or straight but nothing in between. Studies that ask "Are you gay/straight/bi/queer?" alongside "have you ever been attracted to/had consensual sexual activity with a member of your sex" show that many, many people who answered "straight" indicate that they've had a sexual attraction or sexual encounter with at least one member of their own sex - up to 15% and more (depending on if they were male or female), if memory serves.
And, when you do like this website does and ask people to rate how gay and how straight they are, you come up with different results altogether. A great many people will say they're a little bit straight or a little bit gay.
So, many studies - like this one I suspect - just report on people who self-identify as GLBTQ. So 1.5% of the UK identifies as GLBT or Q. There you have it. We're left in the dark as to how many people are closeted, are straight-but-curious, don't adhere to social labels, and so on. Oh, and closet-cases are much more likely than the rest of the population not to answer these surveys at all. So are people who have been persecuted because of their orientation.
And, as joyboytoy mentioned, 3% did not respond to the question. There are any number of reasons why they might not of, but it's not a stretch to suspect that the same societal forces that keep guys on the DL calling themselves straight affected that 3% in one way or another.