Welcome to the party, I have a similar investing profile to you and started about 6 months ago averaging about £50 a month. I have written a little about my first few investments here,
I have been wrestling with exactly what your talking about. I intend to write a post about it soon when I get 5 minutes but essentially I am trying to think about some of the general stock categories I think will be important in the next 20-30 years and research some etfs that I think have a good spread across those companies e.g. robo, emerging markets.
There are some fairly standard ways of categorising stocks but I have really found they tend to be too generic for me. I wrote a few up in a spreadsheet a while ago and had a think about what % of my portfolio I want in each.
Ultimately as you say it’s your choice! But £50 a month is plenty, there are loads of ETFs less than that and no harm in waiting a couple months to buy any more expensive ones that catch your eye.