That's simple these days, just use the betting exchanges, but thousands of profitable systems exist for both backing and laying. The bookies only make 1% ROI, and then have to pay 10% tax out of those profits, so they need to bet £Millions to make anything decent, and a bad meeting like Cheltenham or Royal Ascot can really deplete yearly profits.
Form memory, you needed an 8 figure sum in the end to win £1
Here's a simple system that has profited for decades, back all horses 1/5 or less.Longest losing run was 3 in the early 90's.