10% in UK when we're under 4% of global market cap (IIRC) suggests you're pretty optimistic that we won't continue to stagnate. Hope you're right!
The reason is that I keep seeing information suggesting the UK is undervalued and the US overvalued. That's the primary reason I'm thinking to overweight the UK.
Secondary reason is that it's the home market. Most default pension funds overweight the home market considerably and I'm not sure the reason for it but if there is a reason then it could be valid.