Debenhams in Ilford. May be at other stores too, I don't know.
All the Thomas Nash 'Ultimate' shirts are three for the price of two.
For some reason, the black one with a faint grid is £5 by itself - perhaps a glitch?
Anyway, the receipt below is dated from more than two weeks ago, but I went back the other day and bought a whole load more - almost one of every colour and can confirm the offer is still on, though it seems you need to group them by price - i.e. the deal works when all three shirts are the same price (e.g. two £10 shirts plus one £12.50 shirt doesn't work, but three £12.50 shirts does).
No mention of it on their website though, but here I have overall a dozen shirts for the price of 8.