I can't see why they'd care what your reason was for applying for a new passport - and giving them money. The reasons above are quite valid to quote to them (having to carry 2 passports around or fork over extortion money to IND). You could also throw in the fact that you'll shortly be doing a lot of travel and will need lots of new visas etc (e.g. Indian visa wastes 1.5 pages - 1 with the visa and .5 with the entry/exit stamps). You don't want the hassle of trying to get a new passport within months of arriving in the UK. Or, another common reason is that you need a second passport so that you can travel while you're sending one off for visas etc.