Here in my Somerset back garden I am looking at an April mean temperature of around 9.5, which is a little below average; but not as bad as some years - such as 2013 when it was 8, although cold nights do skew the mean at this time of year.
The GFS for this area over the Bank Holiday weekend is not too bad, giving us 16/17 most days, which tends to equate to 18 in my sheltered space. The Camborne ascent for midnight is showing some signs of subsidence, so I'm still optimistic for the plants to grow, given the 110mm of rain we have had this month so far. The dahlias are appearing already, although they are in tubs which tend to warm up more quickly.
Stay positive.