I thought the theming at blackpool was as good if not better than Londons. Certainly the inter scene theming is better.
When you consider the size and scale of the London attraction, the theming is much more widespread.
Blackpool has some good theming (like the bit in the woods), but I don't feel it's consistent as much as London.
The Castle Dungeon is very cheap by contrast.
I actually think it's a shame it's moving. I really like The London Dungeon. It is, by a mile, my favourite of the four I have been to (London/Castle/Blackpool/Edinburgh).
I could be completely swooned by the move though, if they get a big budget dark ride.