Northumberland - easy choice, driest place in the whole of Northern UK, great bouldering, even some wet weather options. Disadvantages is it might get a bit samey, only 2 sheltered crags if it's windy, could run out of skin.
Lakes - lots of variety in a compact area, easy to escape to Northumberland if needed, good time to go when it's not rammed with fucking tourists. Disadvantages is the real classic stuff is well spread out and some areas might be a bit damp / bleak in the shade of the hills.
Scotland - some great bouldering and amazing lines (Torridon Reiff Ruthven Glen Nevis), beautiful situations, will beat the midges, some rain shadow options. Disadvantages is it's hugely spread out, the weather can be a lot more variable, and rain shadow options can seem many hours away.
Really the sensible thing to do is to check the forecast the day before and make your plans then. There's enough hostels/bunkhouses in all those areas that will be free at that time of year. If it's dry in Scotland go there. If not, the Lakes. If not, the County. If it rains in your first choice head East and/or South as required.
