Green’s Dictionary of Slang

Barclay’s (bank) n.

also Midland bank
[rhy. sl. = wank n. (1)]

masturbation.

[UK]C. Wood ‘Prisoner and Escort’ in Cockade (1965) I iv: hoskinson: Been for a slash en I? blake: Long enough for a Barclay’s en you?
[US]Maledicta IV:2 (Winter) 224: Barclay’s Bank is rhyming slang for wank, and wank is US jerk off.
[UK]Roger’s Profanisaurus in Viz 87 Dec. n.p.: Barclays rhym. slang A wank; five finger shuftle; hand shandy (qv). Barclays bank – wank.
[UK]B. Dark Dirty Cockney Rhy. Sl. 21: I got so bored at work today I popped to the bogs for a Barclays Bank. [Ibid.] 81: Midland bank wank.