As this has already happened, CALL THE AUTHORITIES. It’s not the dog’s fault after all. Carry a big stick. I’m not sure about using mace (legally?) in the UK.**
Meanwhile you might consider altering your route for now.
** I googled and found this –
“British lawBritish law has always been very strict about who can own or move firearms, ammunition and explosives, but this is even more evident now with the continual threat of terrorism. All firearms and explosives must be licensed and there is a total ban on offensive weapons such as flick and gravity knives.It is also against the law to import: high voltage electric stun gunspepper sprays, CS gas canisters and other self defence sprays… “