Depends on your budget for bed?
Trying not to be snarky...but just googling double bed will bring up hundreds of options that's not fb marketplace.
But for perhaps a more helpful view:
There's the specialist bed retailers - dreams and Bensons for beds, for a start. They'll be pricey(ier), but you can go in and choose one, with an assistant who will offer recommendations, help make sure you get the right mattress, offer disposal of your existing bed and delivery & assembly.
Other common retailers - argos, ikea, dunelm will have reasonable quality double beds with levels of cost for different budgets. They will have little to no advice as to what's good/bad, but should at least be able to tell you if the mattress you've chosen matches the bed you want. If you have some questions regarding features, then you might need to research or ask here. They'll deliver, but assembly might not be available, nor disposal.
Less furniture centric retailers like the range or b&m may have some of the cheapest options, but quality and other services like delivery or recommendations on mattress may be debatable.
There's a few online retailers, like wayfair - but they might be hard to order from if you're not sure what you're getting.
If you're not sure on what you're actually looking for in a double bed...then think if you need storage, and what you want it to look like.