Hi, we have this problem here in Portugal - horrible!!!
Salty (he´s nearly 1 now) is not allowed out in the garden for another month or so since we have them in the pines in and around our property
I found the site below good for info, especially the comments - scary
http://www.mythankyoupage.com/pets-and-animals/dogs-and-killer-processionary-caterpillars-in-portugal/ Also,
https://facultyweb.cortland.edu/fitzgerald/PineProcessionary.htmlThe season when they come down from the trees to dig into the ground for pupation is spring, but exactly when depends on mildness of winter and spring
Also, beware windy days during winter as they can get blown off the branches
They are also not very nice at all for people either - I spent most of last summer with hives all over as I was clearing the garden and the hairs stay active for years apparently and the earth and overgrown garden must have been full of them - it was torture and doesn't help that I have the skin befitting of a Ginge
However, I think in the UK its not really anything to worry about, even here for anyone not living in the pine forests its fine, but well worth noting if you plan to travel with or without dogs
Not everyone knows about them, even out here - I have seen many photos of silly backpackers lying on the floor alongside a long procession in singlet and shorts!
Edit - it seems you have the Oak version in places up there too!
http://www.naturalhistorymag.com/features/21756/lethal-fuzz