It estimates a 10.3% increase in sales of vegetarian foods over the next 5 years, with market value rising from £799.7m in 2012 to £882.4m in 2016. Key Note suggests that this growth is driven by health, environmental and ethical concerns.
You can read the summary here. The full report costs £985.
To put these figures in perspective, note that, according to the IGD (see here), the value of the UK grocery market as a whole in 2011 (ie. including non food items) was £156.8 billion.