Is it too late to prune a plum?

Hi. I know plums should be pruned in the summer when it is dry. Well up here in Aberdeenshire it is finally looking like summer and is forecast to be dry for at least a week so I was going to prune the plum tree, an established Victoria which is probably about 15 years old.

But then I did some reading and apparently this is too close to autumn and I risk disease if I prune now, not to mention depriving the tree of nutrients before the leaves drop.

Would I be ok to nip off some of the smaller branches that need to go? I don't think there are any/many thick branches that need to go.

Also, up here if I prune in July then surely I lose fruit? 

As with many Victorias we get glut and fallow, so I thinned lots last year to try to get more balanced crops each year but this year has seen a very small crop. Does pruning play a part in this, and if I prune in July next year should that help?

Info suggests pruning late may to July but as we are quite behind seasonally up here I think may would be a bit early.  Last year and this, June has been pretty wet, and life has thrown lots of stuff at us this year so I haven't had the time to do jobs like this....and we're off on hols for a week tomorrow.....

Any thoughts welcome