I take it that's life time cover OP?
That does seem high and a hell of a jump in one year.
My dog will be 8 in April, has had a few operations - nothing major or terribly serious thankfully - and been on treatment for an ongoing condition that thankfully has been under control for some months now but has flared up several in the past.
I'm with Pet Plan, life time cover, and pay just a bit more than you at just over £64.28 per month and that was with a rise this last year. I'm okay with that as I must have claimed several thousand off them by now.
My excess is £100 and I'm covered for up to £7000 per year. I also get loads of other things on top like complementary treatment, holiday cancellation etc, and of course third party.
Sounds like you're not getting a great deal from JL now, even if it was good in the past.
In your shoes, with your dog's great health history and no pre existing conditions, I'd be looking at other insurers before my dog becomes a senior, when things might get pricier and it becomes more difficult to get a new insurance company to insure him.
I know it's no quite the same, but I've been with the same house insurance company for many years, never bothered to change as it''s only ever gone up by a small amount. This year I was about to renew when I realised that they'd doubled the premium in one fell swoop from last year.
Needless to say I cancelled - they weren't interested when I rang them - and got a fantastic deal with a new company. Sometimes it really is worthwhile to have a look around.