when do i plant out dahlias?

I normally wait till the frosts have gone before planting mine

I agree. You don't say whereabouts you live in the UK but here in Yorks it would be well into May. If you are further south could be earlier.

I'm in N. Yorks as well Celandine but here in Scarborough we are sub tropical - lol. I plant mine in mid-April.

John

if you are in yorkshire back end of may unless they will have foliage protection from other plants beside it. i'm on the edge of the wolds and we too have a microclimate sarraceniac. snow here 2 miles down 4-5 degrees warmer. lol.