You can do that, but it will cost you both of your inbound/outbound days, since you will be travelling in the UK on 2 days.

Additionally, there are currently no Eurostars that serve Ashford.

To avoid using both inbound/outbound journeys on the way out it would be wise to leave the UK on the same day. For example going to Lille and then taking an early train to Bordeaux. It would look like this :

- Eurostar London St. Pancras - Lille-Europe 19:34 - 22:00 30€ reservation (passholder quota)

- overnight

- TGV Lille-Flandres - Bordeaux-St-Jean 06:15 - 12:02 (loads of intermediate stops) 10-20€ reservation

It's faster via Paris but then you must transfer stations (RER/métro not included) :

- TGV Lille-Flandres - Paris-Nord 08:12 - 09:15 10-20€

- TGV Paris-Montparnasse - Bordeaux 10:11 - 12:14 10-20€

Unfortunately you'll have to go all the way to St. Pancras. That's how it currently is due to Brexit and unlikely to change in the next years.

You could catch the overnight ferry between Newhaven and Dieppe, although times are less than ideal.

Last thing : Eurostar trains must be booked well in advance. It's a very popular route and there's a passholder quota. Capacity at St. Pancras is also limited due to extra checks required after Brexit so they can't fill up morning trains. When's your trip planned ?

EDIT : by taking the first Eurostar to Paris you could be in Bordeaux by 13:14. The other option is to take the last Eurostar to Paris at 20:01 and overnight there -> early arrival for sure in Bordeaux (08:14) !

As others have said there are no longer any Eurostar service from Ashford.

You can get to Bordeaux in a single day if you take the 1104 Eurostar to Lille. You should be able to reach this with an early train from most places in Dorset I guess. This means you use only one UK travel day.

But you can stay overnight in London and use two travel days if you wish.

Alternatively, you could take the ferry from say Poole or Weymouth (at your own cost) and then use your pass from there.