Cute dress, flattering style especially with vertical stripes. It looks like an A-line, mini dress, not sure it would be a swimsuit as I don't think they would usually have a zip. The label states 'Short', so it is probably just a petite womans sizing. What is the length from shoulder to hem?
It is definitely not 1950s, but could be mid 60s or possibly early 70s . Here are some points on the St Michael label that help with dating;
St Michael introduced Size 10 in 1962, and an R in a circle, replaced 'regd' in the mid 60s - so it will be mid 60s or later
Made in Britain - in 1970 'Made in the UK' was phased in (it could still be early 70s as it was being phased in)
By 1973, size was shown in a red circle
Does the back of the label show anything, or are there any other labels? They could provide really useful info.
e.g. washing symbols - the first washing symbols were introduced in 1966
CA (Canada) code - St Michael introduced these codes on their labels in June 1972
Inclusion, or lack of these symbols/codes, could help narrow down the date further.