Ask them if they already have employees in a similar position (so the company is aware of tax, labour laws, visas, etc. etc. etc. and isn't going blindly into this).
Without the experience, it may be quite difficult both for you and for the company to estimate what an appropriate salary would be. Let's say it is a position where you and your employer agree that $80,000 a year would be perfectly fine if you worked in a Boston office. It's then very hard to say what would be appropriate if you worked in an office in Glasgow or Munich.
There's tax laws, exchange rates, cost of living, the cost of moving to a place temporarily (because you don't want to emigrate, right? ) that need to be considered. If the company is experienced and a decent company, they might say "we would pay $80,000 if it was in Boston, and we think £xx,xxx to be appropriate for the same job in Glasgow". You might then accept if you think $80,000 is right and hope they are honest and experienced with their foreign salary, and if it turns out that salary means poverty in Glasgow, then you would complain.
If neither you nor the company have experience, then the correct salary is a tricky bit. Or if it turns out that the UK throws you out after three months, that's bad for both you and the company and you should both figure out what to do beforehand. There is the "expatriate" website for that kind of problem.