en.wikipedia.org/…/Drought_in_the_United_Kingdom

Droughts are a relatively common feature of the weather in the United Kingdom, with one around every 5–10 years on average. These droughts are usually during the summer, when a blocking high causes hot, dry weather for an extended period.[1]

rmets.onlinelibrary.wiley.com/doi/…/joc.6521

Overall, the drought of 1834–1836 was the most intense SPI-12 event in our reconstruction for England and Wales. Newspaper accounts and documentary sources confirm the extent of impacts across England in particular. We also identify a major, “forgotten” drought in 1765–1768 that affected the British-Irish Isles. This was the most intense event in our reconstructions for Ireland and Scotland, and ranks first for accumulated deficits across all three regional series. Moreover, the 1765–1768 event was also the most extreme multi-year drought across all regional series when considering 36-month accumulations (SPI-36).

en.wikipedia.org/wiki/1928_Thames_flood

The 1928 Thames flood was a disastrous flood of the River Thames that affected much of riverside London on 7 January 1928, as well as places further downriver. Fourteen people died and thousands were made homeless when floodwaters poured over the top of the Thames Embankment and part of the Chelsea Embankment collapsed. It was the last major flood to affect central London, and, along with the disastrous North Sea flood of 1953, helped lead to the implementation of new flood control measures that culminated in the construction of the Thames Barrier in the 1970s.

This produced the highest water levels ever recorded in the Thames in London.

I don’t think that regular water level records on the Thames were kept, and it sounds like flooding problems were apparently worse from the sea, but it doesn’t sound like the present day sees the worst flooding or low water levels.