I believe it was to do with the continental container traffic gauging as I recall it happened around the time the Channel Tunnel opened and of course well before HS1 was built.
But that might be erroneous.
There are two classic routes across Kent which are cleared for SB1C swap bodies and W8 containers: Dollands Moor - Ashford - Maidstone East - Swanley - Shortlands Jn - Nunhead - Factory Jn - Kensington Ol - Wembley EFOC. The alternative ( diesel hauled only due to weak power supplies) is Dollands Moor - Ashford - Tonbridge - Redhill - East Croydon - Clapham Jn - Kensington Ol - Wembley EFOC. The bridge building was one part of the Channel Tunnel upgrading of Kent. Improving track quality was a major and much needed improvement and finally the upgraded power supply for Eurostar and class 92 workings.
Some bridges via Sevenoaks and Orpington / Bat and Ball were also replaced, but I suspect that was due to the structures needing repair or replacement.
Via Sevenoaks was not cleared for SB1C or W8 , but a few containers went that way and left scars on the lining of Sevenoaks tunnel.
It's almost academic these days there is so little Channel Tunnel rail freight. Much of the container and swap body traffic now runs over HS1 to and from the new terminals at Barking. What does run via the SE domestic routes is steel traffic and conforms to W6a profile.