I'm not based in the UK, but frequently I can find a few more surveys by going back and clicking on previous e-mails. Doesn't make any sense, but it works.

ETA: and I may have found a decent alternative to e-Rewards- learned about it in a FaceBook feed. (Yes, Big Brother is alive and well.) So far I've completed enough Surveys to get 1,000 points; they're similar to the e-Rewards ones, many from the same survey companies. There's a delay of couple of weeks while they actually check your responses to see if your answers are credible and until then you can't see the rewards. I'm at that stage, having accumulated over 1,000 points with about 1/3 approved. I was waiting to see what the rewards actually are, and then found this review- basically a variety of $10 gift cards or $10 to PayPal for 1,000 in points, so well worth it. No invitation needed.

https://www.surveypolice.com/branded-surveys