Base miles are the miles that keep you ticking through the winter - and being in the UK, the winters are pretty crap. So two tactics are used.
Basic base miles. They're done consistently out in the cold and dark, Z3, spells of Z4/5, the key is consistency and burning through the winter. Time to test yourself against the elements, enjoy the changing seasons, fight the winds and rain.
Virtual miles with the bike on the turbo trainer, PC logged into Zwift, in the garage, consisting of structure workouts using Sufferfest and other apps. Shorter in duration than the road miles, more Z4 and Z5 bursts, and massive massive MASSIVE amounts of bass.