I also know about the relocation costs and possible reimbursements, etc. but again this is not the point. These costs are a once-off, and though it may influence your salary initially (i.e. more relocation bonus, less pay) it should not do so in the long run.
The point I was trying to make (and perhaps badly so) was to not focus on the salary only - these things vary more than you may think. And that it is easier to move jobs once you are in the UK, i.e. (1) get a job with work permit and preferably relocation bonus, and (2) reassess your position after you've been in the UK for a year. It's a lot easier to have a work permit transferred from one employer to another than it is getting one in the first place.
Relocation costs also generally don't impact on performance bonuses in the larger companies (who are also more likely to go through the trouble of a work permit), as the costs are met from separate budgets within the business. The cost to a company of using an actuarial recruitment firm is likely to be more than any relocation reimbursement anyway (recruiter: 10%-25% of your first year's base salary, vs relocation budget of max 3,500 pounds - you do the math).
By the way, the idea of "cost to company" reimbursement is also quite uncommon in the UK - most companies measure base salary only (this is what will usually appear on your offer letter). Any additional stuf like pension contributions or bonuses are usually specified separately and not in your employment contract (makes it easier for the company to make changes without having to re-issue employment contracts or side letters to contracts). So if you receive an offer of say 35,000 pounds pa this will usually only reflect base salary, excl any bonus / 13th cheque, which will be additional to what is in the offer.
I agree with Kail's point about contacting a recruitment firm to get a feel for "typical" pay level, however, good luck for getting a straight answer out of them. You're likely to get something similar to my first post (if you're lucky).
Good luck with the job-search, and hope you find the offer you are looking for.