The only advantage of the ISA or pension is the tax benefits. If you're not a UK taxpayer, then don't bother.
The question is then whether to keep that £50k as cash, or to invest it, and I think that depends on what's next for you. If it's money that you genuinely don't need to touch for years, then put it in a world index tracker (eg Vanguard's VWRL, or one of their life strategy products). You can deal directly with Vanguard, or go via an online share/funds company e.g. Halifax, Hargreaves Landsdown, Selftrade and many others. If you think there's a fair chance you may wish to access that money in the next 5 years, then I would keep it as cash - even though you are effectively losing it little by little because of inflation. For example, if you might want that money to buy a flat or house if/when you return to the UK (a house is first of all a place to live, not an investment.)