To Glasgow via Manchester
Why the trains don't go beyond London is a mystery - Eurostar even built special shorter trains to cope with the shorter platforms in the UK. Sadly these special 'North of London' trains are now being used to run shuttle services between Lille and Paris, due to the massive demand on that route. Apparently these trains are only being leased by SNCF, so Eurostar could get their hands on them pretty quickly if they decided to run a North of London service. 'Nightstar' sleeper trains were also built, but these never saw service as they appeared just at the same time as EasyJet and Ryanair did, so they were sold to Canada.
Another question you might want to ask is why you can't catch Eurostar from Ashford to London - I can get onto the London->Paris Eurostar at Calais, and I can do the return journey, but I can't get onto the Paris->London train at Ashford.
People asking about the time it took the UK to build HS1 might be interested to know that planning permission is a lot easier for the government to get in France than in the UK, and that the French on the whole are much more into megaprojects than the UK (see the Milau viaduct and the Pont de Normandie for good examples - I would think that had the UK government been building those, the solutions would have been very different).