Besides the start and end of the trip, I can’t see there being need for too much money. I might need some to buy snacks and bits and bobs on the journey, but day-to-day I won’t need much.
Sabah uses the Malaysian ringgit (MYR), as opposed to Brunei dollar or Singapore dollar used in the Bruneian parts of the country.
Exchange rates are currently 1GBP to 5.47MYR, or 1:5 for a quick calculation.
You can’t buy MYR here in the UK so I may just take USD and exchange when I get over there (1USD: 3.33MYR).
Afterthoughts:
I didn’t bother with USD in the end - I just took GBP and changed it while I was there. I changed about £300 and hardly spent any money, what with food being so cheap. I’d say £200 would suffice for this trip, unless you really wanted to go all out on souvenirs. ATMs and money changers were plentiful in the cities, though - we changed our money in Centre Point in Kota Kinabalu.