If you want to do a flip, buy it cash do it up then mortgage it, you'll have then bigger chance to get all your money out what you invested and move on to the next one.
I'm not the fan of interest only btl mortgage.
You can also only release equity when your fixed rate period ended on the mortgage.
Trouble these days every man and his dog at it, the market is saturated with stock that doesn't sell, because estate agents, property course providers talk the bullshit and people buy into it.
Yes HMO is a great thing, what's even better is the licensing fee for it which is slowly getting introduced by every single council. Your responsibility is much bigger and with HMO more agro as well to find trouble free tenants as you have more units under one roof and them units have to get along with each other. That's when it kicks in that you not just a landlord but a social worker, mediator, etc.
Commercial is also great, till you end up without a tenant and after the 3 month grace period you have to pay business rates plus all the standing charges. Saying that last time we had an empty unit we let it in 24hrs after a quick paint and new carpets.
Getting mortgage on flats above shops and restaurants take aways is getting more strict too.
I had a 2bed flat offered a month ago for 140k, lease extension 22k, refurb 30k, market value according to EA 2-210k, in reality next door struggling to sell for 176k with slight room for improvement and that's in home counties.
All I can say is do the maths not twice but ten times, a small profit is no profit, you need a fair wedge to cover everything from EA to solicitor to contingency.