I don't think the nurses can make a referral to a specialist - I think it has to be the doctor.
As richi says, jot down the points you want to make so that you don't forget them. If your GP's surgery is a group practice and there's a choice of GP's to see, choose to see a male or female doctor - whichever you're most comfortable with. The GP's surgery where I go lists the doctors who work there on their website, showing their experience, how long they've been there and their areas of medical interest. You may be able to get a GP whose area of interest is gastrointestinal medicine. If not though, don't worry, all GP's will be able to recognise that you have a genuine condition and will be able to make a referral to a specialist.
If there's a particular GP you've seen in the past, maybe about a different matter, and you liked that GP, why not book an appointment with the same one? If not, what I do is go for GP's who've been at the surgery some time rather than those who've recently joined. You can usually find out from the surgery's website. That way I know I'm seeing someone established and relatively experienced, rather than a locum who may be there on a temporary basis. I've had a couple of less than positive experiences with locums who were not UK qualified.
Sometimes however, the newly joined practice members are merely young and more recently qualified. You can sometimes get further with someone like this as their training is more recent and they can be more enthusiastic and keen to get to the root of the problem. If other people in your family go to the same GP's practice, perhaps they know which doctors are more approachable, or perhaps you have friends who do? Having a look at the website can be a start.
Apart from that, just try to relax. As richi says, they'll have seen thousands of bottoms! Just stick to the point, say what you've been experiencing and for how long, how much it affects your life, and ask to now be referred to a specialist.
Let us know how you get on.
Whoops, just seen richi's new post - I keep cross posting!