I don't think you understand mortgages very well.

You know you can remortgage or sell your house before it's paid off, right?

If I pay in £500 a month. Likely, 20-80 of that goes to interest. I'm gaining £480 in equity each month.

Which I can gain access it at near any point with very little penalty.

You're focusing very blindly on your single point of "the deposit". Which is the hurdle everyone should focus on however possible.