1 I'm not a citizen of the UK, but I follow a lot of people over there. Since the precedent of how their financial institutions handle situations often affects how US financial institutions think of situations I've decided to talk a little bit about their banks on my blog.
2 I do think that by-and-large companies should be able to choose to refuse business with whoever they want (I dislike most top-down civil mandates on association) but extrajudicial coordination to restrict basic services to hamper legitimate political organization sows mistrust and fear for those wanting to participate in the political process. It's a disincentive for participating in public life. Extrajudicial coordination to lock people out of organizing would not be tolerated with utilities or travel companies in the UK ― I'm confused why financial institutions are getting a pass. Banks should not encroach on the responsibilities of the state ― should stick to being banks unless they want to devalue themselves.
3 Debanking adds risks and mistrust. It adds risks and mistrust not only with dealing with banks that are imposing the sanctions, but also when dealing with the currency itself if the group that are imposing the sanctions handle a significant percentage of the 'everyday transactions' of the currency for physical goods/services. No one wants to deal with a currency if the institutions that control the payment-processing for it extrajudicially coordinate to put ad hoc sanctions on their clients. Money loses worth if there is a question if it can be easily spent in practice. Less people wanting to risk dealing with UK financial institutions will cause the £ to devalue itself.
