This is quite a common shape, seemingly very popular in the 1930s and made by a number of firms. Yours is possibly by Stevens & Williams/ Royal Brierley, I had a very similarly shaped example in their rainbow glass, shown below. I believe Webb, amongst others, also made this type of posy vase.
There were also many pressed pieces made in this mushroom shape such as Bagley's 'Elf' posy and Jobling's 2595 pattern.