April and May here in London is still quite cool - usually around 60 to 65F average.
Average temperatures by month for London UK
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
5°c 5°c 7°c 9°c 13°c 15°c 18°c 18°c 15°c 11°c 7°c 5°c
41°f 41°f 45°f 48°f 55°f 59°f 64°f 64°f 59°f 52°f 45°f 41°f
The above are only 'averages' - we do sometimes get quite warm weather here in London when it might get up to 70 or even a massive 75F.
London is one of those strange places which enjoys a temperate climate - neither blazing hot nor freezing cold - in other words, extremes of weather are very rare. Yes even snow is rare.
Most of the time London is overcast (grey) - but we get less rain here than they do in Rome. Hard to believe but actually true.