I want to order an iPhone 5s but I live overseas in England. I know I can at the unlocked version but since it's GSM, will it work with any SIM card here?

Order it from Apple UK if you want your iPhone to work with British carrier's 4G network. The US iPhone version (if that is where you are thinking of ordering from) does not support the bands used by British networks.

Also the warranty is limited to the country of purchase (in the case of the UK it would apply to EU countries as well)

Finally, Apple US will not ship to the UK so if you want to order it from Apple US you need a US mailing address.

Note that only the online and retail Apple Stores sell unlocked iPhones in the UK. Do not buy an iPhone from Carphone Warehouse; they'll often claim they're "SIM free" but the iPhone will lock to the first carrier you use and will not be usable with any other carrier after that. Nor can any other reseller in the UK sell a truly unlocked iPhone.

Regards.