You can get 5% on cash in current accounts at TSB (tho’ limited to 2x£2k) and at Nationwide (£2.5k for 1yr). Also there are 6% regular savings accounts to be had from First Direct, HSBC and Halifax, although Ts+Cs apply. All subject to tax though.
Overpaying on your mortgage is never a bad option as the savings are tax efficient, but you may struggle to borrow back the cash in the current environment.
Forget cash ISAs, the rates are barely better than inflation. And premium bonds are a con trick.
Individual shares can be highly volatile over the short term, so this would be a pure gamble.
Funds are more diversified, but with stock markets all near all-time highs there are few tempting options for a short term punt. Fundsmith (as mentioned above) is hugely and justly popular and has done very well up to now, but it simply can’t continue upwards at the same rate it has been. Personally, I would (and have!) put it in something that’s predominantly Euro-denominated, like Threadneedle European Select. Why? QE has caused the Euro to be sold heavily meaning you get more for your £, and QE should also tend to fuel asset price rises in the short to medium term. No guarantees tho’!