Isn't this criminal here? Under the computer misuse act for taking the website down and under the theft act for blackmailing you for the £5,000.
This is yet another case that proves the point about not mixing business with friendship though unfortunately.
Do you have a coherent, written partnership agreement?
If the answer is "no", this is going to be a brutal lesson as to why you want a written partnership agreement.
As the shareholder is a director it won't be theft, the company can do what it likes with it and each director acts with company authority subject to the powers granted in the company documents.
Blackmailing would probably still be relevant though (I assume).
Well, the answer will be "no" as the OP has a Limited company not a partnership.