I'm currently living in Cambridge, UK. My company contributes 5-15% of the salary to the pension. I'm planning to permanently move abroad in 3 years. I can opt out of the pension, or stay in the pension plan for now and take money out when I leave. In terms of tax, fees and benefits, which method is the better one?
Assuming that the "5-15%" means that they contribute 5%, and then they match whatever you contribute up to an additional 5% (so 15% possible total) then there would be no reason at all to opt out of the "free" 5% unless it is a trade-off versus another benefit that you would rather have instead.
For the remainder, how much free cash-flow do you have? In other words, can you afford to contribute 5% in order to get that additional 5% match? If you can, then you should at least consider it. Any fees you pay will be small in comparison to the contributions you get from the company. You won't pay tax on the money you put in and whether you pay tax on it when you get it out will depend on the situation at that time, but remember you're effectively getting a return of 100% on those contributions from the company matching, so it's unlikely to end up costing you.
In terms of what to do when you move abroad, you could either leave the pension pot here to accumulate until you are at least 55 (you can't withdraw from a pension before that age), or you could transfer it abroad. More information on those options is available from the Government.
Note I am not a financial advisor or tax advisor so please treat this advice as worth what you paid for it. Ideally, consult a real IFA!