Vets fees are the same whether you are insured or not. Insured pets maybe more likely to be referred to specialists who are more experienced but more expensive for similar treatment.
Check with your vet if they will do direct claims with the company you are planning to go with. That means you only have to pay the excess and they claim the rest from the insurers. They won’t do it with all as some pay out very slowly.
Personally PetPlan are choice for the dog and provide good cover an usually good to deal with. Look for lifetime cover, at least £4k in cover and a with all insurers check the small print.
Ps don’t over exercise it either, springers can have elbow problems.