Does anybody know if you can marry your fiancé while he is still in prison and I'm in england?? Would I be eligible for a fiancée visa??

I don't know for sure, I can tell you that to get visas in Canada and the US you have to show that you have sufficient income to support the fiancée until the fiancée can apply for permanent residence.

Since he's in prison his income is 0 so in North America the visa application would be rejected. Also even aside from his 0 income I don't know for sure but I assume that a felon (especially one still in prison) wouldn't even be allowed to be a sponsor.