Pick a fund. Enlist the help of a financial advisor to do this if you have no idea what to pick or contact a company yourself and pick yourself out of their products. They come in lower, medium or higher risk categories. But all shares funds are essentially gambling. You could lose the lot. But probably won't, so long as you don't pick a higher risk fund and you're prepared to have the investment as a long term plan. The funds are made up of shares in various companies, the fund manager chooses what, and basically gambles with your money to try to get you the best return on your investment. They buy and sell shares in different companies as the prices rise and fall, to achieve this. Somehow, somewhere, the fund company aims to make some kind of profit themselves (or else they wouldn't do it. I've no idea of the details though), they charge you a fee for managing your fund too. They take that fee out of your investment.
It's not something you do if you want to take the money out next year and you'd be devastated if it was half it's current value. If that's the case, put it in a savings account where regardless of how shite the interest rate may be, at least you won't lose the original capital.
If you're happy to leave it alone for eg 5yrs and you're not going to think it's the end of the world if the worst happens and it's all gone, then invest it.
It's not all or nothing, either. They're currently worth £3k? Let's say in a year they're worth £3.5k but you need a replacement car. You can contact the fund, tell them you want eg £1500 and they'll sell some shares for you and send you a cheque, which you bank and then go out and buy yourself a "new" old wreck to drive. You can leave the remaining £2.5k in there to hopefully increase again over time.
You keep UC informed of the value of the shares whenever they ask. The fund company can send you a statement any time you ask. They won't care if it doesn't go over £6k. So if you see it's at £5.5k maybe take £500 out and spend it. If you go over £6k your UC amount will be decreased. If you go over £16k your UC claim will end.
Disclaimer - I am no kind of financial person, I just inherited some shares once.