Nothing to stop you walking Southwark to Canterbury to Dover. Where you go from there gets a bit trickier.
You could try Canterbury to Winchester to Portsmouth & a ferry to Bilbao / Santander.
No one in Shoreham by Sea will take you to A Coruna these days though it was a popular route in the 15th Century. In 1423 Thomas Attenhalle would ship you cheerfully, and his minders who would walk you to Santiago and back (just to make sure you did come back). The boys in the Yacht Club these days tell me that France is as far as they go and that not so much since Customs & Excise started getting interested in payable duties again.
What might work is Nottingham to Westminster in memory of Eleanor of Castille
https://lookup.london/charing-cross-history/ and then a flight from Gatwick to A Coruna
Edit to add: I have a current inkling to walk from home to Gatwick / Coruna - Santiago - Ferrol -Coruna / Gatwick. I no longer collect Compostella so I'm not worried about confusing the PO. I am a bit gruntled that there's no one in the Yacht Club with any sense of the absurd