Sorry to be so blunt, but you already failed at step 2.
Remember, market sizing isn't about coming to the exact right answer insomuch as it's about assessing your ability to have structured thinking and ability to think reasonably. You didn't do this :/
Wrong approach - You cannot just randomly guess what % of your total pop is hairdressers in order to figure out how many hairdressers are in the UK. This is not breaking down a problem into logical segments in order to make more reasonable assumptions. You literally just said "ah, maybe x% multiplied by the total population!"...that's not strategic thinking...sorry!
Ok, so, how to we fix this? Well, we have to look at what ways I can break this down into more reasonable steps/assumptions!
So, let's thinking about:
- # of people in the UK
- # of haircuts per person per year
- Think monthly
- Break down population into reasonable segments that affect frequency of haircuts
- Throughput of an average barbershop/hairdresser
- Think hourly and expand out
- Calculate # of shops needed (by # of haricuts and throughput)
And viola! Much more reasonable to make these kinds of assumptions!
Hope this helps, but feel free to message with any questions!