salary?!! i wish.
very few places offer a salary. it's usually freelance basis - £35-£40 per flight hour for restricted FIs and £40-£45 for un-restricted. no paid holidays, no benefits whatsoever.
some companies even expect you to clean aircraft etc for free when you're not flying (!)
if that sounds like a good hourly rate, think about how little you'll fly over the winter, or if you want to take 2 weeks holiday a year, or when you have some days with just two 30 min trial lessons.
some places offer a monthly retainer (about £100-£200), but not many.
as was said earlier, you better have a way of making money on the side if you're planning to be an FI in the UK.