I had my chest measured recently and it was a revelation.
Go to a department store and get some Fantasie bras as a guide - they are fairly true to size. The bandwidth is important; you need to hook on the outside hooks and it should be sturdy once fastened - you shouldn't be able move it around easily/have a lot of 'give'. You need to let out the straps so that there is no riding up at the back; they don't bear the brunt of the weight of your breasts, which should be taken by the band.
You may need to fluctuate according to the material the bra is made of.
Once you have that, you should establish cup size. The outer wire must be well clear of the breast tissue and pointing up to the armpit. The inner wires must be able to sit flat against your sternum. Lean forward and jiggle to position yourself in the cups. The material over the breast shouldn't dig in; neither should you be able to get a finger easily in there/see any major saggy bits.
I would go initially for a 30DD or E (on basis that you are smaller than me - I am a 14/16 - and got fitted as 34F or 36E). I found the Fantasie Belle style very supportive.
EugenesAxe Fantasie bras run approximately a cup big in most styles- and the Belle full cup is huge! Whatever size you wear in their styles, you would need larger in the majority of other similarly priced brands.
I have just been and bought new bras (tried loads on) following on from the many threads on here. I am the most comfortable I have been in years. (I also have a bosom for the first time since my milk came in when breast feeding!!)
Fair enough married! I bought some others based on my Fantasie fitting and they were fine, so that's why i said. Everything else stands. The fitting person said sizes vary by brand; she seemed to think Elle McP came up weird.