It depends on the type of sizing being used. DDD is only in US sizing and is the same as a US F; however, a UK E is the same as a US DDD. You really just need to look at the brand to know what sizing they use. UK brands have a better size range (some go down to a 28 band size even up to J cups or larger), so you may have the most luck going somewhere that stocks UK brands you can try. Nordstrom is generally a good bet (they carry down to a 30 band and usually know what they're doing).
US sizing: D, DD/E, DDD/F, DDDD/G, DDDDD/5D/H
UK sizing: D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, L
Measuring guide:
http://www.thinandcurvy.com/2010/10/how-to-measure-your-bra-size-correct.html
Fitting guide:
http://brastop.com/fitting_room.aspx