Four Counties do a policy called Travel Plus which covers adventure activities normally not covered on standard policies. It covers all sorts of activities including "Motor Cross" and "Motor Track days (amateur not competitive)". I've used it and had the broker explicitly confirm that it covers bike track days.
£24 for 5 days or £123 for an annual multi-trip policy for track days.
£33 for 5 days or £165 for an annual multi-trip policy for motor cross.
www.4counties.co.uk