A lot of English seaside towns restrict dogs from some or all of their beaches during the main summer season, typically running from around May to September, though the exact dates vary from council to council. These restrictions are local byelaws, not a single national law, so there's no one UK-wide rule to memorise — it genuinely differs town by town.
The usual pattern, where it applies, is a seasonal exclusion on the most crowded central stretch of beach, with a nearby, less central stretch remaining dog-friendly year-round — a very similar structure to Switzerland's lakeside Badis, just applied to the coast. Local council websites publish the specific beaches and dates each year, and it's worth checking the destination town's site directly rather than relying on a general rule of thumb.
Scotland and Wales have fewer blanket seasonal beach bans overall than England, though local restrictions still exist in places, so the same check-before-you-go approach applies regardless of which part of Great Britain you're visiting.