A deposit is the part of the price you pay yourself; a mortgage covers the rest.
5% is the smallest most lenders accept. Rates are higher and choice is smaller, but it gets you in sooner. Plenty of lenders lend comfortably at 5%, so it is a real starting point, not a last resort.
10% opens up more lenders and better rates. This is a common target.
15% or more usually means the best rates on offer.
On a £250,000 home, 5% is £12,500, 10% is £25,000 and 15% is £37,500.
Remember the extras: solicitor fees, survey, moving costs and, in some cases, stamp duty. A few thousand pounds on top of the deposit is normal.
The Ready Score deposit part gives full marks at 10% of the price, because that is where lender choice opens up, and half marks at 5%. Many lenders lend comfortably at less than 10%, so a smaller deposit does not stop you buying.