When (how far in advance) do British tourists book their summer
holidays? Can you find some statistics?

I suggest that you contact ABTA (The Association of British Travel
Agents) for hard statistics.
Their current press kit states that:
"It has been a ‘late booking’ year and it is unclear whether this
pattern will continue into 2003. Although the market performed well in
January and February 2002, many people left booking their main summer
holiday until late June and July - after the Queen’s Golden Jubilee
celebrations and the World Cup."
Certainly the most holidays are sold during the months of February &
March.

An article from The Observer suggested that there were still 2 million
unsold holidays in July:
http://www.observer.co.uk/travel/story/0,6903,750631,00.html

Good Luck

Links
ABTA
http://www.abta.co.uk

ABTA trends for 2003
http://www.abtamembers.org/press/kit/trends.htm

ABTA corporate enquiries
corporate@abta.co.uk

ABTA Annual Report (PDF)
http://www.abtamembers.org/press/kit/annualreport.pdf