This is where statistics can be misleading, just because indoor coverage had increased 12% donesn't necessarily mean that the number of calls made in those locations will increase by 12%.
We're talking about nearly 1% of all calls total, the vast majority of calls probably aren't made in indoor former blackspot areas. As 800 coverage continues to grow and the priorities get tweaked I think we'll just see a gradual improvement. I stupidly bought an LG v10 from Germany, wish I'd bought an official Three phone, if the V20 comes out in the UK I'll consider it if it' supported by Three.
That slight extra 800 could come in useful, I have tested 800 on my LG G3 with TinT and it does make a difference, I have made VoLTE calls and been able to get 4G in a hotel room where I would have otherwise only had 3G (by forcing it).