UK south coast you have mostly muddy bottoms, which the Rocna is perfect for. Check if it fits your bow roller though (they have PDF templates online that you can print and make a cardboard anchor for checking from). Their sizing guide is good and will tell you what you need - looks to be a 15 kg Rocna as you're slightly over 4 tons, and a bigger anchor will generally hold better and if you're on weed/grass, it will penetrate better as the tip is longer.
We've never dragged on the UK south coast with our Rocna 25. Not even during
this episode. We did drag once in a blow in a rare muddy bottom in Menorca because a pair of discarded pants was caught on the tip
There is also the Vulcan 12kg. The Vulcan is basically a Spade copy (by Rocna) and supposedly performs as well as a Rocna, but without the roller bar, using ballast instead for positioning. It's a good weight compromise and the lack of roller bar means it will fit your bow without any problems. These factors may well make it the optimal choice - I had to modify our bow roller for the bigger Rocna as the roll bar was clonking into the pulpit.
If you do go for the Rocna, please do not anchor the way you have always done, but actually read the manual on how to correctly deploy and set the anchor. We've had a boat drag onto us in the night with a perfectly good Rocna because they were using it completely wrong.
If weight is a problem, Spade makes an aluminium version too, which I've personally seen perform extremely well. They're quite costly though.
The Delta is a now outdated plow style design and won't perform nearly as well in mud.