At a guess I'd say a 2:2, but it's pretty difficult to calculate these yourself since, as you say, the grade averages have a weighting applied to them each year, and actually some of the core modules may carry more weight than electives. Some universities use mean averages and some use median, which can produce slightly different results.
Most universities have some sort of classification calculator that you can access which lets you put in your module results and run it through the university's algorithm. If you use Moodle or some other e-learning site then it'll probably be there, or you might be able to find on the uni's website just by googling.