Direct debit into a deposit account with your bank? Online accounts often pay competitve rates. That'd be the easiest.
I wouldn't worry about interest too much, the rates are rubbish at the moment. E.G. If you found an account paying 3% and put the full £5k in now, you would only get £100 odd after tax, but you are only putting in £125 a month, so you wouldn't get anything worth mentioning relatively speaking. The banks are having it all their own way again!!
If you were saving for the long term, Tax free is worthwhile if the rate is competitive. The idea is you keep money in tax free savings and transfer it from provider to provider at the end of each year (if required) to get the best available rate at the time. The annual allowance is not great, but over time, you'll amass a significant sum of money on which you'll get tax free interest. A very nice, especially if you are a higher rate tax payer and the rate is decent.
I wouldn't bother with term accounts on small sums, just look for the best rate (if you are saving long term).
Premium Bonds are hopeless as a savings vehicle. You won't get any interest and the prizes are unlikely to come anywhere near what you would get if you stuck the money in an average savings account. Of course, you could win a million pounds and this is the benefit for the cost of seeing your invested sum depreciate year on year. In summary, if you can afford to have a flutter, but want to keep your bet, go for it. It's much better value than the Lottery and the odds are probably far better stacked in your favour (but still very very long). I would always consider Premium Bonds as a "nice to have" in addition to the majority of your money which should be invested elsewhwere.
Of course, if you have large sums to invest, things become much more complicated. This is advice is for people with a few grand's worth of savings to look after, not an investment portfolio. For that you need a professional advisor.
the account has to have £2000 to be opened 
