There is now a daily service from Paris to Barcelona in Spain which does not involve a change of trains at the French/Spanish border as previously necessary. The direct service from Paris now takes 6½ hours. a saving of over 3 hours on the original times.
This does mean that in theory you can travel by Eurostar/TGV from London to Barcelona by train in 10 hours, albeit with the need to get across Paris from Gare du Nord to Gare de Lyon. leaving London at a civilised hour in the morning, and getting to Barcelona in time for supper!
Travelling by train from London to Barcelona became so much easier with the launch of the new high-speed rail service from Paris to Barcelona. During the peak summer season in 2018, there will be a later departure too, also routing via Paris and reaching Barcelona late evening. On certain days – but not in winter – there is a good alternative route from London to Barcelona with one easy change of train in Lyon. It’s no quicker than the Paris route, but it saves the hassle of having to change stations in Paris – see Rail Europe
By Train TGV from London with Rail Europe