Ian wrote:
Will want to hire bikes for a few days while on a US holiday. If I do, will
I be charged lots for a helmet and for other extras like repair kit/spare
tube, so its worth bringing them on the plane from home in the UK?

Helmets don't take up much room if you pack them full of clothes.
Ditto tools. (Note: pack your tools in checked baggage. TSA stole my
tool kit in April, even the plastic tire levers and patches. I tried
to get them to explain the inherent danger of a spoke wrench or chain
tool, but they were stupid. Knitting needles, no problem. A 4" long
8/9mm wrench: into the trash, might be dangerous.)
Also bring your shoes and gloves, if you wear them. May as well be
comfortable on a rental bike.
Depending on your destination, it might make sense to buy a bike
and then sell it back to the shop. Some places are very amenable to
this. (I used to work at a shop near a campus, and we'd "sell" bikes
for the summer for $25, buy them back for $10. Great rental fee.)
Where are you going?
HTH
--Karen M.