Your "low" p/x value is based on what your car will be worth at the expected time of arrival for the new one, not what it is worth now.
If you have a fixed price trade-in and that price is stated on your order form then that should be that - no changes. You have a contracted value for yours. In situations where the car takes a lot longer and the dealership gets stung, they can apply to VW UK for subsidising discounts to make up their unexpected residual losses on your p/x.
My R is almost 3 months later than originally anticipated and the salesman hasn't asked me for a penny extra.
Some dealerships are cautious about extended VW waiting times - the local rivals to my preferred dealership group don't do fixed p/x prices on GTI/GTD/R, they promise to maintain your equity.....Say you had a car they said was worth £15k to them right now and your VWFS settlement cost was £13k, you have £2k equity. So you order your GTI and it takes 5 months to come, you've made 5 more PCP payments and now your settlement is £11500 - they would give £13500 to maintain your £2k equity.
Doesn't work well if you own your car outright though.