You can get 1,250 bonus Avios for signing up to the UK TopCashback site here

Although it's the UK site, you can pay out any cashback using Paypal, and it could be useful to have an account anyway because they sometimes have higher rates/different options than the US site.

You need to spend £10 (about $13) with any retailer featured on there to trigger the Avios, but note that it's spend £10, not earn £10 in cashback, so is really easy to do.

I can't remember seeing a publically available UK TopCashback signup bonus before.