For a UuA from WT+ to CW you should be paying the difference in carrier surcharge - it isn't a tax difference. The carrier surcharge applicable is route specific, generally though the longer the route the more the surcharge. For example starting from the UK the additional surcharge in CW compared to WT+ for a return to NYC is around £106, so the cost of doing a one way UuA would be 10k avios and £53, and 20k avios and £106 if you UuA outbound and inbound flights.

Also, in all cases any changes in taxes, charges for customs, immigration, YQ, airport service fee, passenger service charge which have happened since the original booking was made will be taken in to account, but these are usually minimal if they even exist.

So to answer your question, for LAS £63 per person extra carrier surcharge each way sounds about right (I haven't checked the exact figures for LAS). The agent was right, there are no new/extra taxes to pay.