This man knows Belize, follow his advice.I don't know pounds, sorry. The national currency is the Belize dollar. $2BZ = $1US. The USD is widely accepted and often preferred.
Rough costs are hard to answer as you can spend a lot by staying at a resort or very little by staying in a youth hostel. My guess is your budget is somewhere between the two.
I would budget $100/night hotel but be pleasantly surprised by the lower rates in the "slow" season if that's when you go. Groceries are limited and expensive but cheaper than they are in Hawaii. Good restaurants will probably run about $25 - $30 and up for dinner per person, but you can find plenty of hole in the wall type places and get a good meal for $10.
Things change considerably if you go out in the countryside. One of my favorite things to do when I'm travelling through is stop off at a house with a "food" sign. Usually sit on the front porch and eat a delicious home cooked meal of stewed chicken, rice and beans washed down with ice tea. Typically $5.
On dollars and dollars, US cash spends everywhere. Credit cards work at some places, but not for that $5 plate of stew chicken, rice and beans....