Very nice bits there,that type of helmet was also used by RN gun crews and also Royal Marines for riot/crowd duty. plenty of chinstraps complete on Ebay and should only cost you a couple of quid.
The last hat is a Combat Cap,50s era from memory and unfortunately missing its oilcloth lining and leather sweatband. Funnily enough only just looking at a complete one on Ebay,reasonably priced at £15 as a smallish size(larger ones go for a lot more).