I think expectations of £14k might be a little optimistic.
There are currently three 2013/2014 manual white 3 door GTI Golfs for sale on Autotrader - all being sold by dealers. The most expensive is £15.5k (just under 29k miles, leather and panoramic roof). The other two are around £14.5k with 45k - 50k miles - both performance models.
Those are retail prices, so will include the dealer’s mark up for profit margin, cost of providing a warranty and cost of preparing the car for sale (service/MOT, VED, valet etc.) which could account for £3k - £3.5k of the selling price.
Assuming those retail prices are realistic, I’d say on a good day you might get £11k to £12k, but the price you get would obviously depend on how you’re planning to sell it (e.g. part exchange, selling privately, selling to a car buying service).
I’d forget about original box with VW flask / pen - that’s likely to have been a freebie to you or a previous owner, courtesy of the VW dealer when they sold the car (I’ve got one, along with a collection of VW umbrellas, pens etc. from previous VW car purchases
).
