I agree with Fleisher that there are not any software comparisons out there worth reading which would help you make your decision. So why not download limited node demo copies, from REI, CSI, Robobat and compare for yourself? I would add one more for your consideration, Risa 3d, although I'm not sure if it offers british standard design options.
In just a couple of years, software products can change quite a bit. For example, Fleisher says he uses Staad for dynamic analysis of pumps, but we had problems with Staad just a few years back on a similar application - Staad shell output would not give us forces/moments at joints, no constraint options forcing us to model with rigid dummy elements, and poor meshing options to name a few of the issues we had. Sounds like some or all of these limitations may have been resolved
My point is not to criticize Staad, but to point out that all programs have their good pts and bad pts, and most of us don't have the time to keep up with the changes and enhancements made to these various products unless a vendor gives us an on-site presentation or we attend their seminars.
But with availability of demo software, you can do a hands-on evaluatation to compare and decide for yourself
