Not quite sure why, but their relative productivity per worker has been dropping since 2007 compared to all other developed nations. [1]
With low productivity, it doesn't matter if you have sweet trade deals - you're still going to decline eventually.
One, UK is growing at about the same rate as other developed economies. This is quite remarkable because, if you look at Europe, 100% of these countries have significantly higher unemployment.
Two, productivity growth has slowed everywhere after 2007. The UK is comparable to Western Europe, slower than Germany, faster than Italy, it depends heavily on what start and end date you pick. This isn't particularly surprising given that we essentially outlawed the most productive sector of our economy after 2007.
Three, the UK had no financial boom in the 2010s, the US did, and the EU did...let's just wait until the debt has to be repaid to see the full score on whether that has worked out economically.
Hopefully the EU can be reformed to go back to a trading bloc, with a liberal visa regime for member states, rather than an idealistic United States of Europe model, punctuated periodically by brutal austerity.