The most useful starting point in any garden parasol buying guide is matching canopy size to the area you actually need to shade. A 2m to 2.5m model suits bistro sets and compact patios, while 2.7m to 3m works better for standard outdoor dining. Larger rectangular or cantilever designs are a better fit for long tables and corner seating, where a centre pole tends to get in the way.
Base weight matters as much as canopy size, especially in changeable UK weather. Check whether the parasol comes with a base, whether it needs slabs or weights separately, and whether the pole diameter is a match. For fabric, polyester is common and perfectly practical, but look for decent UV resistance, a vented canopy, and a finish that will cope with light showers rather than fading quickly.
It is also worth checking parasol height and clearance before ordering. Some larger models give plenty of shade but sit low over a dining table, while others suit lounge seating better, where you want more headroom and easier movement underneath.