I found a couple more sites there in UK offering tubers. :
However, I don't have any experience at all with them. I'm just too far away!
I use a German site as a source for what is being traded at present:
They offer addresses of the main European providers. ( I now don't understand how this will affect after Brexit
In my experience with growing dahlias its important to start with a reliable source that provides healthy tubers. It takes time to understand who's who in the market, but its really worth reducing variety and spending slightly more in quality...
I start my single types from seed. i.e Bishop of Llandaff. I got those seeds from Thompson & Morgan. I had blooms the first season on very dark leafed short bushes. I still keep them and I've just finished dry storing ( and dividing) by clumps for the next season. I've also grown own crosses( larger varieties) from seed and have kept some that have caught my attention. In every case I do prick my seedlings when they reach 2 pairs of leaves. They go into individual pots (mine are pastic bags 20 cm in diameter) filled with a third organic compost plus manure and 2/3 sandy soil. They stay inside the greenhouse until around the beginning of December when they are acclimatized outside and finally placed in the ground. The exact timing depends on the temperatures and general weather. Normally my dahlias start blooming mid-January onwards and the larger dinner-plates February. They bloom until first frosts.
Since I try to grow them organically, I provide large ammounts of horse manure and also chicken bedding from our farm. The larger varieties are ravenous feeders and require extra fertilizing during the growing season. So they get at least once a generous addition of manure as a side dressing.
Please feel free to ask further questions.
PS I've been gardening dahlias already for about 25 years.