The castle class was bought because they had a flght deck, it was the main reason the BN was so interested in it. One has to consider the Chinese effect here, any major ship procurement, like Frigates and destroyers will probably come from China or will be modernized with Chinese weaponry. ( the castle class are two have Chinese C-703 SSMs).
The Bn isn't interested in any British frigates, its either going to order more F-22Bs or the Ulsan.
As for corvettes, there was a lot of talk about the Milgem class, and BD might just order it in 2015, ones its more matured.
Turkey and South Korea have become big players in the growing defence market in BD. Bangladeshi navy crews have received extensive training on =board Turkish Type-209s and the Bn are likely to procure second hand Type-209s from Tukey.