The 14 day distance selling relates more to new vehicles, because for used vehicles, you can return within 30 days.for any reason minus reasonable admin costs.
This is the law, so they can't say it doesn't apply.
Also with used car, if it develops a fault within 6 months, notify the trader and they have to offer you the option of a repair, replacement or buy back of the vehicle.
Again, this is Law, so you don't even need a warranty for this and a seller trying to hide behind a warranty claim is probably trying it on with naive buyers.
Buying used from a trader, my advice is buy, get it properly inspected by an Independent Merc specialist, fault codes checked, underside trim and wheel arch liners out, and if it's got problems, take it back.
Any warranty is mostly useful for catastrophic unexpected disasters is months 7-12 of ownership, so don't pay for or worry about warranties 6 months or less. The reason they are pushed for buyers to buy/given for free up to 6 months, is for the benefit of the seller to minimise risk of costs or to try and hide behind.