With £1,000 to spend you should be able to buy a good second-hand shotgun set up for a left-handed shot.
Whether you choose a side-by-side or over-and-under is a personal matter, but if most of your shooting is clays, an over-and-under might be best.
Choose a gun which is open-bored and avoid screw-in chokes as they can cause trouble. For most of us, choke is a solution to a problem we don’t have!
I suggest you go to a shooting ground with a gunshop and try some of the dealer’s guns, and take a lesson or two. A good coach will ensure the gun fits and will take a note of any necessary measurements to be altered.
It is a buyer’s market at the moment and some gentle haggling may well produce a competitive price.