As has been said, interest rates are pretty low at the moment, which is good news for people with mortgages but bad for savers. It depends on whether you want to save the money long-term or just want to make the most of it over the next year or so.
If you are looking at long-term savings and are likely to return to the UK for a period of time, I would look at ISAs (or pension if you are a UK tax payer currently) to make the most tax-efficient savings in the long-term. Otherwise, for short-term gains, most accounts you will get around 2% APR at most. Moneysavingexpert and other sites can give you a good idea of whats available. Yes, you can just leave the money in an ISA or savings account. I wouldn't worry about not being able to do a regular saver as when you look at the equivalent APR over a year, they generally aren't much different from putting a lump sum in on opening a normal savings account.
If you wanted a flutter, you could put half in an ISA / savings account and half on the stock market (in an ISA if you want), noting that you can lose it all and need to invest time and effort researching stock. You can invest as little as £500 with a broker like H&L and the AIM market stocks can fluctuate massively (think 90% losses or 100% gains in 3 hours). Funds are generally a good idea over the long-term because they buy stock from lots of firms so the gains and losses tend to average out.
Or you could look at peer-to-peer lending like Zopa, but after the fees, defaults and tie in period (generally 3-5 year loans) I have found I have only made a little more interest than keeping the money in a bank. So if your primary aim is to make money in the short-term its not really appropriate.