Is it wise to be 100% equities with no diversification?
I think that's the wrong question. 100% stocks is wise for some people but not for others, and it comes down to your personal goals. Will investing in 100% equities meet both your financial and emotional needs?
Here are two relevant charts. They're for the US stock market, but should be similar enough to the UK to be educational.
If losing money for more than a decade or watching your portfolio drop nearly 40% in a single year is unpalatable, then 100% stocks may not be for you and there are better options (here's a
collection for comparison). Likewise, if you have a specific retirement year in mind and want more certainty in your financial path to get there, diversifying outside of stocks can help you with your time-sensitive goals. The same goes for something like saving for a college fund.
On the other hand, if you have a long investing time horizon, the personality to handle the ups and downs, and no real time-sensitive goals for the money, then 100% stocks may be a perfectly fine choice. It's also a great foundation for new investors just getting started. If you can't decide where to put your money, buying something like VTSMX today and taking the time to learn how to diversify new money in the future is a great choice. There's no need to wait until you have the "perfect" portfolio plan to start moving in the right direction.
Basically, one cannot know if a particular portfolio is a wise choice until they first understand themselves and how various portfolio options may affect their personal life goals. If you're not sure about either, then start simple and build up as you learn more.