Ok to be clear on pricing be sure which cruise is being quoted it may be different cruises.
I will nearly always put prices to match any agent or source that i mention so in the case of the reply to TravelMandy it was NCL US in $ and NCL UK in £ US prices will nearly always have tax and fuel added, on the 30march case that is $170. If I quote an agent as well I usualy put their prices in as well
Only some agents discount NCL us but this may be for a limited internet only service
As for conversion that is more dificult because we don't know what rate you will get. As a rough guide use $2-$1 and if the price is within 5%(cheaper US to UK) then look for the proper rate if you use a credit card find out their charges usualy upto 3%.
I use nationwide Credit card which has no fees/loading so will get the market rate on the day I get charged for a booking so I use
http://uk.finance.yahoo.com/currency-converter?u (2.0273) to give me a more accurate £ price for the $ bookings but when the difference is more than 5% or even 10% why wory about the detail on the last few £ I will be booking it anyway by then. ( on a £1k spend 2-1 against 2.0273-1 is $27.3)
NCL Pricing on any cruise up to end of June will be in or going into sell out mode so will drop unless they get short of cabins.
Book when you are happy with the deal.
Booking through the US is safe enough(no ABTA or ATOL) but NCL charge the CC not the Travel agent if a bit worried book direct with NCL for more piece of mind but a slightly higher price on low grades or a lot on the high grades.
I you think you have a cruise identified try
http://www.cruisecompete.com to get a few agents to give you a quote.