It’s one of those questions John to which the answer will vary between individuals. You don’t mention HYPs specifically but in order to be able to construct a suitably diversified portfolio in my strategy requires a minimum of 15 sectors, so with one share per sector that’s 15 times whatever the economically viable minimum investment per share is for any particular broker, having regard to costs.
I doubt that the £2,500 you mention would be anywhere near adequate and would suit more like 1-5 shares perhaps, thus increasing risk. Not that there’s necessarily anything wrong with taking on greater risk if the person understands what they are doing, it’s just it would not be an HYP as I define it and this is an HYP discussion forum, not a general investment forum.
Also I point out that if you are charged £90 per year, that 4% yield on £2,500 would be just about swallowed up by the annual charge and leaving next to nothing to reinvest.
An alternative for such a small sum might be a fund, of which there is an enormous choice, especially for a more risk averse investor.