I agree with Phil Rush that rates will come down -- for all the various reasons he mentioned. However you should consider two things:
a) When you remortgage to another lender than Nationwide, the whole process might take up to 12 weeks. So just before Christmas might not be fast enough -- and I understand the new Nationwide deal at 6.2% will lock you in for another 2 years? So the right time to look for something better than 6.2% would be today!
b) There are deals today which are better, eg between 5.2-5.8% (HSBC and their 1stDirect division), but they are usually capped at LTV between 75-80%. With your valuation of loan/value = 148K / 180K = 82% approx, you seem unfortunately to be just above the threshold. On the other hand there are usually fixed fees attached between 1000-2000 GBP, that would be another 0.75-1.5% up front, or simplistically calculated over 2 years, add 0.4-0.8% to the cheaper rates p.a. So these fixed fees pretty much kill the advantage for you, as your mortgage isn't very large.
Conclusion:
Yes, it seems your mortgage advisor has got it right for now. Maybe Nationwide has a better deal until Christmas, probably internally they won't need the 12 weeks to shift you to another deal.
Nevertheless, don't you think you can find some savings you've made somewhere, in order to push your LTV another 7% lower, to below 75%? In your case, some 15,000 from an inheritance or an old ISA or something? Just in general 75% is a bit of a threshold below which there's much more competitive rates offered. Also think about whether your new deal should be a capital repayment mortgage rather than interest-only. The interest-only deals make me personally uneasy.
About me: Timing-wise I am in a similar situation as you, remortgaging from the Halifax as per end-of-year. I probably will go to First Direct at 5.49% tracker, which makes sense for me because the fixed fees don't play such a role for me, as my mortgage is much higher than yours, but my LTV is considerably lower at only ca 60% so I am in the range for these products. Also tracker means if BoE ease their base rate, my rate goes down with it. Added bonus is that 1st Direct offers this together with an off-set facility so I don't pay interest on whatever credit balance I have in my current account (daily calculated). Just thought I'd mention that.
Anybody has a better idea let me know ...