- Train Swindon to London Paddington, 130km
- Xtracycle Paddington to St Pancras, 5km
- Eurostar London St Pancras to Paris Gare du Nord, ≈500km
- Xtracycle Paris Gare du Nord to Paris Gare de Lyon, 5km
- Paris Gare de Lyon to Valence, ≈500km
That'd be a 2300km round trip, about 10 hours each way.
I don't know how much the train is London to Swindon, but Eurostar would be about 100-150€ return, and Paris-Valence by TGV about the same. Plus any extra charges for transporting a tank with pedals. So about the price of the bike again, all in all.
This a great bargain for someone a bit nearer though.