These people are pretty good, and you would have full access to the server, so you could install anything you like. It means you would have to install the forum yourself, or get phpBB to do it for you (which they will do).
I've been in touch with them, and they seem very helpful. They suggest that you head over to their site and have an online chat with their support person. I've been talking to someone called Adrian. If you want to ask specific questions you can email him at: [email protected]
but I decided to take the plunge with Siteground after all. I even managed to get my first choice of domain name free. So I now own both the .co.uk and the.com They also give 900 gigabytes of monthly traffic, which was one of my main concerns. I hope that will suffice!
I am at the moment configuring the BB which was a doddle to install. I will then run the two forums side by side during the changeover period so that members have plenty of time.
As I am not the most techie person in the world I will keep you informed of the pitfalls I come across so that someone else may benefit.