You need to do it on an investment platform. I only use Hargreaves Lansdown (HL), I'm happy with them but I have only ever used one other (Fidelity, which I got fed up with, I like HL, but I don't have experience of other platforms. But HL is a well known and respected investment platform. You can invest in a standard fund and share account, or in an ISA or SIPP (additional charges apply to these 2 though, it is £200 a year for the SIPP and £45 a year for the ISA, the fund and share account is free for shares, bonds (which the Wasps bond is) and etfs).
Here is a link to their website
https://www.hl.co.uk/ You would have to register and then you can invest, it costs £11.95 per trade to buy or sell (less if you do quite a lot of trades in month, which I have very rarely done). If you need any help just ask.
Currently the Wasp bond can be bought for about 51.5 pence, if it goes well and reaches maturity on 22 May 2022, you would almost double your money in capital gains (on top of the interest received and capital gains on individual corporate bonds are tax free),
but obviously there is a significant risk, but it could still go wrong and they might still pay more than 51.5p in the £. The headline interest is 6.5%, but that is on £1, if you bought for say 51.5 pence, the annual interest would be approx 12.6% (6.5/.515).
I normally do not touch risky investments returning anything near what this bond hopefully will, but I have always thought at every price that they were value (if you can live with, and handle the risk and accept that it might not work out).