Sweet blackberries - suggestions please for allotment varieties

[newspud9]

While its quite easy to pick wild blackberries in this part of the country (south east), to be honest they dont come close to the sweet variety available from supermarkets at the moment- and which I believe are from the UK.  Grateful for any suggestions on sweet varieties that can be grown (and when) on the allotment as well as soil conditions and care needed to get the best fruits.

Many thanks for all the suggestions.

[newspud9]