To jon87……Your GTI hasn’t dropped in value at all.
You are confusing purchase and trade in levels.
A dealer will look at a margin of £2,500 to £3,000 on any car, depending on lots of factors (too many to list actually).
Likewise, the trade-in offer will be based on many factors including assessment of the customer themselves.
So, factoring in all that plus “a few” months older and a few more miles and another owner on the log, I think your trade-in offer is pretty good.
If you were to buy your car today from the dealer, he would be asking much nearer what you paid for it.
If you want an independent valuation, try Motorway or similar.
However, this feeds into another forum topic “Where have all the Golfs gone?” and leads me to speculate whether prices for good 7.5s might not actually be on the way up ……apart from at VW dealer showrooms, obvs 
Would anyone like to give a dangerous opinion on current and future values of 7.5 GTIs?
Exactly this, something you just have to accept, what you pay is not the value of the car, its value of the car + the buying protection and warranty protection.
We bought a MK 7.5 R last September with list price of £25000, would say be lucky to get £19500 trade in now, and £20-£21000 private sale. I would still have to pay the list price of £25000 now for a decent MK 7.5 Golf R, they not really dropped since last September.
I knew this going in though, I had a trade in, which managed to agree a better price than expected, so VW approved was only way was completely comfortable with, and still preferred over pay maybe £2000 less for some of the private sales that was seeing at the time, with very little comeback and warranty, (though you could add a VW warranty latterly if car has FVSH)
Personally, realise as soon as drive car off forecourt its worth about £2000 less immediately, but its likely car going to keep for 3yrs at least, so £ are worth the peace of mind, and when it does come to sale, time elapsed has you take a "is what it is" when value is given for likely trade in.
Cars have always been a depreciating asset, you spend the money for the pleasure, thats the investment, and personally not regret a £ spent to date on any of mine,
Be in the market for a late MK 7.5 next year to replace the 128Ti, and do think the cars at £25000 now, will be more £22500 by then, think the MK 8.5 will send more MK 7.5 to the market place and soften market just that smidge more.