How to reclaim IRS tax - help requested

I was lucky (and I do mean lucky) to win a $2 trifecta bet while at the Breeders Cup in Santa Anita, USA.

I got taxed at 30% & given a form W-2G as receipt. I am a UK resident & I'm back in London now.

Does anyone know how to reclaim this tax - any help would be much appreciated as my googling skills (or lack of them) are failing me.

Thanks if anyone can help