My understanding is that north of London, third rail electrification is rarely used instead, overhead line equipment is the standard, which is also what the current Eurostar trains rely on. One of the biggest issues is that the new Eurostar trains are built to the larger European loading gauge, meaning they’re too big to fit on most existing British railways. At present, they can only operate on the purpose-built High Speed 1 line. Upgrading the rest of the network to accommodate these larger trains would be extremely expensive, as it would require widening tunnels, platforms, and bridges. However, older Eurostar trains or other rolling stock built to the smaller British loading gauge could still operate on the existing rail infrastructure.
obviously the other large infrastructure additions at wixams for passport control , longer and secure platforms would in my opinion make it far to expensive to be economically viable.