This is going to sound very snarky, but it is what it is.
Everything about that wedding is out of line. And the Bride-to-be is delusional. Expects people to be paying for everything and bringing the materials for the wedding and on top of that expecting presents? Utterly tacky and embarrassing. If you don't have the money you do simple and personal and small and that's it.
Sweetpeach, I don't think this will be a fun weekend, but I hope it somehow will be.
I actually run my own honeymoon registry site. Its just easier to mention where you are registered and have the guests decide if they will find something from the register, or choose something else to give you. My husband and I used my own registry service because it was easier to do that than to have items or money shipped. And besides, we DID already have the basics.
But registries should never be forced upon others, and saying just give us money is tacky tacky tacky. There are better ways to put it.
granted, the fact that its your DH's best friend means you are pretty stuck on this one. But my suggestion would be to accept it for what it is and do the least. And if it were anyone else, there'd be 20-40 quid and an apologetic "It'd be more, but after all the costs of the weekend, we are tapped out. But I know our presence here, sharing your day means the more to you, so we made sure we were here all weekend." and leave it at that.
I'd say more but it'd involve very colorful language

Best of luck to you, Sweetpeach!