YANBU, it's gone up loads, there are wild differences in the cost across the country and it was way way more expensive than we'd expected!
My DB had a registry office do earlier this year, gorgeous room, plenty of people, chandeliers, access to a beautiful garden afterwards for photos, cost £100.
We had ours a few weeks later, different county, cost about £400 more, far fewer people, no chandeliers!
I have no idea why it's gone up so much. People I know who did it in our local country 4 or 5 years ago spent £100 and it's ballooned since then.
Having it in another venue and having the registrar come to you is going to be even more, especially on a weekend.
I agree you should find a nice registry office somewhere else in the country, have a night away if you can! All the local county websites show the prices, which are cheapest mid week during office hours, then more on a Friday, much more on a weekend, and shit loads after 7pm.