Because the bed is higher than the pavement level, it's probably quite well-drained which will help the dahlia survive the winter if you leave it in the ground. My Bishop's Children dahlias come through the winter in the ground here and I'm a bit further north than you (sandy well-drained soil).
Quote Dan.....I'm in the Midlands - Northampton - Irises tubers survive the cold where I am - does this mean Dahlia would be ok - I think these are just beautiful and would fill a border with them if I thought they would survive ok?
I suspect Dahlia belonged to some one else and was just spotted in passing by DAN WILSON
Either way, I think it would survive in the ground in Northampton, and long as the soil is reasonably well-drained so it doesn't get waterlogged and then freeze.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Perfect - good advice all thank you. I see no reason not to raise them a little to assist and its got to be worth a whirl as they are so fabulous looking - thank you.
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I suspect Dahlia belonged to some one else and was just spotted in passing by DAN WILSON