The single best thing you can do before framing a poster is confirm it is a real A-size. Every PostersPosted print is, which is why any standard UK frame fits without a bespoke mount.
Buy A-size, not 'poster' size
All PostersPosted prints ship at true A-sizes - A5, A4, A3, A2 or A1. That matters because 'poster size' on Amazon can mean anything from 24 × 36 inches (imperial) to A2. Filter for A-size and the print will drop straight in.
Frames we recommend
IKEA Ribba (24×32 for A4, 30×40 for a mount, and A2 Ribba) - reliable, affordable, comes with a mount that lets you display smaller prints inside a bigger frame.
Habitat and Dunelm both stock full A-size ranges including A1, which is harder to find on the high street.
For A5 keepsakes, IKEA Fiskbo A5 and John Lewis's small frames both fit perfectly.
Hanging without damage
For anything up to A2 in a lightweight frame, Command Picture Hanging Strips are the safest option in a rental - they hold well and peel off without lifting paint.
For A1 or heavier framed prints, use a single small screw into a rawlplug. The screw hole is trivial to fill; sagging Command strips are not.