I think you are talking about splitting the booking at Brussels and discussing the fares thence to Cologne?
The DB Advance fare to Cologne starts at EUR19 on the 10.25 train (which is the one with the Ashford connection) but EUR49 in first class.
Thalys is coming up at EUR25/39 which is better value in first because of the freebies, but you've got a 3 hour wait in Brussels outward.
This doesn't seem a huge difference to me - the DB fare will apply to a considerable range of destinations beyond Cologne (potentially right across Germany, I haven't checked for this particular service) - but if you're only going to Cologne that isn't an issue.
I suspect the £160 return is the open return, which is what RailEurope and so on will sell you. I checked for Weds 27 Nov outwards BTW, because of the booking horizon, on the DB and Thalys websites.
The London Special fare referred to by Gerald Fiennes can't be booked online from Ashford (I don't know if Eurostar would allow you to use a LS or Thalys London ticket to join there, as with their own tickets I believe?); tickets for after the timetable change in mid December won't be on sale until mid October.
However, they routinely start at EUR 59 in Standard and EUR109 in Standard Premier/DB First each way, and are almost always the cheapest option, especially going beyond Cologne. BTW There are comparable Cologne fares on the Thalys site, but these seem harder to get hold of.
It's not the advice I'd generally give, but with the restricted choice you have at Ashford, with destination Cologne, and with it being Christmas, I think booking Ashford-Brussels return as soon as you can, and the other two legs separately (DB out, Thalys back?) when they go on sale, might be your best bet.