The law in the UK is very hazy. It basically says it's obscene if it will corrupt those it is intended or likely to see it.
Otherwise what's legal in the US is legal in the UK. Penertration, erections, oral, cum shots, anal, DP, etc is all legal and pretty well trouble free in stills. If you are doing moving images you are going to have to have a sex shop license, obtained from the local council, and these are very hard to get.
Go to a Pakistani run newsagents and look at the top shelf mags, you will be surprised at what is allowed.
How do I know all this? I sell the pictures to UK magazines.
Your biggest problem is where you host from, if you host from the US you are not publishing in the UK. Therefore not governed by the UK publications laws. But steer very clear of kids, animals, toilet, abuse, pain and force related content.
Bottom line ask a lawyer for legal advice before you go forward.