Hello Julie, I am unsure as to whether Malaysia will or will not accept a pre-paid cash card, it would be wise to contact the card issuer for that information.
Luckily, Malaysia uses the same 3 pronged plugs and sockets as the UK, and the voltages are similar also so you will not require an adaptor for this trip.
With regards to the weather, it will be above 20 degrees centigrade when you visit in May, so thin clothing is advised. However, it is likely that you will experience some rain in May so I would recommend taking a thin waterproof coat, one that will keep you dry but one that isn't too heavy and hot to carry around.
I hope this information has been helpful and if you have any further queries then feel free to email me.
Kind regards
James, your editor