Technology companies that interface with banks and credit companies are becoming more common as banking itself is more and more driven by technology.

My question is, relatively speaking, how complex are the systems developed by Simple (in particular), Square, Mint, Paypal etc in relation to interfacing with banks/bank accounts?

They're mostly proprietary I'm guessing, suggesting there's significant IP and talent involved. Paypal in particular would probably be a leader here, aside from credit card companies themselves.

The reason I ask; right now I'm looking to open a business bank account in the UK and the UI's look pretty uninspiring. Given that I'm likely to do a lot of banking online, an independent service that could provide a better UI and importantly, could advise or save me money, would be something I'd consider paying for.