Parcels are priced by volume as well as weight so the price for 15kg might vary according to what the item actually is and how large.
I use Interparcel, though I have not used them between France and the UK, they use a range of couriers and you can choose which service.
The cheapest they offer for UK->France on a 15kg 50x40x30cm package is My Hermes 2-5 day service dropped off to one of their collection points. That is £16.50 (inclusive of VAT) - pretty much the same as your 19€
Going the other way all their offerings are ridiculous though.
Don’t forget you’ll need a CN22 post Brexit