All helmets sold in the UK comply with the same EU standard for safety, whether it be a £20 jobby, or a £100+ one. As with motorbike ones, get one that fits and is comfortable. Your mainly paying for the internals and ventilation as they get more expensive. I have a giro hex for the extra protection to the back of your head / neck. Looking for a full face at the moment
its hard finding anywhere to try them though. Thinking the non carbon POC.
As for cramps, if you sweat a lot it will be a lack of salt. I've got to the end of a ride and my hair has been white and crusty with salt and I've been able to scrape it off my dry face. No amount of food the previous day will replace that. Try some high5 tablet drinks, they are nice. You can also get some good salt tablets from myprotien for a few quid.