Although there a few tour companies that operate boat trips to view dolphins in UK waters (e.g.
https://www.ramseyisland.co.uk/boat-trips/offshore-islands-whale-and-dolphin-voyage ), there are none that allow you to leave the boat and swim with them. (Apart from any other considerations, the waters where dolphins are found around the UK tend to be well off the coast, where sea conditions are often far too rough for safe swimming).
Further, swimming with dolphins anywhere is strongly discouraged by many wildlife conservation bodies. e.g.
https://uk.whales.org/our-4-goals/end-captivity/swimming-with-dolphins/