Does any member know if you can visit a friend in prison in Bangkok

You can visit. It may be best to talk to the Consular Section of the British Embassy for advice about your friend.

you sure can, check out the articles about it on www.khaosanroad.com

Another useful site is Bang Kwang prison. Will give you all the info you need - visiting times, how to get there, what to wear, proceedures, what you can take and what you can't etc.

I believe you only need to contact The Embassy if you want a contact visit, but they are only allowed once a year. Probably worthwhile to call the Consular section anyway.

I've only visited immigration prision and the Special Detention Center. You need to take your passport, with up-to-date stamps and visas of course. They check everything, and I believe in Bang Kwang you need permission from the prisoner's embassy or something like this.