I'm actually not sure where and why do you catch this problem. As you see, on our forum we have iso-8859-1, and if you check the contents of this page, £ sign is not even transformed to an HTML entity.
My admin html posts were not displaying the £ sign at all.
Not sure on this as well. If you have used £ it should be displayed correctly almost everywhere.
Obviously the version of html, php or even the database must be causing it.
I'm mostly sure it's about browser and how it deals with special chars in encoding table. It just "supplies" something to miniBB, which is not liked by it. You may debug the text you get step-by-step; something is definitely thrown out in textFilter
function of bb_func_txt.php. As you see, it already has a lot of commented places to debug the input, so you may try to de-comment them and check at which stage the "emptying" of the string occurs.