What do you mean by bank holiday weekend? This matters, because there's a big difference between 4th of July vs MLK day. If you pick a popular weekend, it's more likely guests have other plans.
Anyway, what matters most is Saturday night. ANY Saturday wedding will be more expensive, because this is what most people want. Sunday and Friday are usually less, but again, if you pick a popular "long weekend" type thing, you may run into trouble with guest plans.