Cash and travellers cheques in US dollars are preferred by banks to British pounds. Outside Dhaka and Chittagong you'll have problems changing pounds. Credit cards are widely accepted at hotels, guesthouses and restaurants in Dhaka and Chittagong, but virtually nowhere else. Amex users can get a cash advance with their card. A tip, or baksheesh , seems to be demanded in almost every exchange, except in the more isolated rural areas.
Most transactions require bargaining, which is considered a normal part of life in Bangladesh.
A rule of thumb is to offer about half the original price and work up. It's worth remembering that a few extra taka are likely to help your bargaining adversary more than they'll hurt you. We encourage you to verify any critical information with the relevant authorities before you travel. This includes information on visa requirements, health and safety, customs, and transportation.