If there is a branch at CW you simply take your book in and ask the teller for an update. This will make a B/F entry in your book. But yes, making a deposit or withdrawal will have the same effect
Agent doesn't help. Sadly some expats living in Thailand think it's the same as using a visa agent to get a Thai visa, where a bribe is paid to an immigration officer. Doesn't work that way with UK visa. All UK visas are processed through a separate department at the British embassy in India. Good luck reaching them!