No, yaySIM does not provide an unlimited data eSIM for the United Kingdom. All of the plans listed for that destination have a hard data cap, with the largest offering 100GB for 180 days.
Travelers who need a large amount of data can choose from fixed data plans such as the GB_50_30 plan, which gives 50GB for 30 days, or the EU‑35_100_180 plan, which offers 100GB for 180 days. These fixed plans can be more cost‑effective for most users, unless the traveler is an extremely heavy data consumer who would benefit from a truly unlimited option.