800 is just part of the jigsaw. With the recent work in Hatfield and Welwyn Garden City, I now seem to get 4G indoors at almost every place I go where it previously dropped to 3G, and yet it's still just 1800.
What I think has happened is more site have been 4GEE'd and as such the power has been tweaked. Maybe 800 has been added too, as there's quite a lot of countryside around, but it's clearly not essential everywhere for a big improvement.
Not all 4G is double speed, but most, and a lot less is 4G+, but it's now pretty hard to get below 20-25Mbps and quite easy to get 50-60, with the 4G+ stuff being around 140-150.
800 is, as said, the frequency of last resort. Definitely need it, but the continued roll out of 1800 and even 2600 is just as vital.