I’d also recommend the FB groups ‘Dog Training Advice and Support’ and ‘Reactive Dogs UK’. Loads of great advice available on both and the former can point you towards decent trainers/behaviourists Who cover your area.
It's incredibly tough. I have a nervous dog who can be great for weeks but then yesterday had s complete meltdown when a retriever puppy looked at him We had a session with a behaviourist which helped me manage ddog ( careful to find one who recognised how frightened he was rather than aggressive/ dominant ;) ) If you can narrow down what the triggers are and the distance from them your dog can cope with then this can help. If you have a bad day then don't worry about taking a day or two off from walking to allow your dog to calm down again. In the days off you can occupy your dog with training at home, as a key thing is getting him to focus on you and trust you to keep him safe.
This may not be quite the same situation, but PestDog is intermittently reactive towards other dogs.
The core thing that has made a main difference is sufficient exercise - which he wasn't getting before I started looking after him. For a while in thought it was all the training I put in, but then I discovered that if he goes more than a day without being walked to the point of exhaustion then he's straight back to his old ways
Our dog is reactive to small dogs, particularly terriers, after being attacked by one. We've worked on training him to do certain things that can distract him when one comes close - 'find it' = looking for treats scattered in the grass, 'jump up' onto walls/benches and 'watch me' = looking at me for 3-5 seconds to get a treat. Basically all things that give him a chance to do something positive and earn reward and praise. Obviously these only work when you spot the trigger in time! If not, I hold on to his harness and we stand still while dog goes past. I don't say anything or try to haul him away, just give him immediate praise and treat as soon as he looks at me once the trigger is passed. Very gradually, he is looking back at me quicker each time rather than barking and lunging. But it is slow progress! I think the biggest thing is I feel more relaxed on walks because I can see these things slowly working and so he feels more relaxed overall too.