The Co-operative Bank is the simplest bank to deal with in the UK. Its call center manned by British operators. Contact numbers are +448457 212212 from abroad or 0800 590483 from the UK. Answering times are typically very small. Operators are not emotionally difficult and they never try to sell you anything.
The online interface is by far the simplest to use among all UK residential banks. The online security relies on a few access questions and a card reader.
Co-operative seem to tick all the boxes of user-friendliness and straight customer service. In this regard they sit in the antipodes of Citibank, for instance.