Plan for the weather
A UK event has to assume rain, and probably some wind. Choose for the worst plausible conditions, not the forecast you are hoping for.
For rain, look for a properly waterproof structure with heat-sealed seams. PVC and heavy-duty polyethylene both shed water well, but the seams are where cheaper shelters let you down.
Wind is the one to respect. No temporary structure is built for severe weather, and the sensible plan is to monitor the forecast and take canopies down if strong gusts are coming. For exposed sites, multi-day use or anything left up overnight, choose a heavier frame such as a heavy duty gazebo and anchor it fully. Our guide on using marquees and gazebos in bad weather goes into this in more detail, and it is worth reading before any outdoor event.
In hot weather, UV-resistant material keeps the interior cooler and protects guests from the sun, which matters more than people expect at summer events.