Thanks for the reply.
As I am only in China for the transfer I did not intend to have any RMB with me nor use an ATM and have any RMB left over just before my departure from XMN.
What do you think of XiamenAir's 3 options? Ignore them all and just get a taxi into the city instead, if so I will have to read up on getting the 72 hour transit visa unless my 2 year UK tourist visa already covers me?
If you really don't want to get any RMB, then probably the sightseeing bus is not a good option for you. It's just that kind of hop on/off bus that connect you with major sightseeing points in Xiamen, that you still need to pay for entrance tickets (if apply for that place) and cost for food/drink (also depend on you). Going out with 0 RMB may not be a good option, withdraw 300-500 RMB from ATM will be a good cover.
Back to the options Xiamen air provided, definitely the lounge will be most simple choice as you will stay in airport without bothering anything, but it will be quite boring to spend half day in Airport lounge as well