Estimates of the number and proportion of foreign registered heavy goods vehicles on roads in England are not available. However, estimated numbers leaving the UK are published in table 1 of the quarterly bulletin “Road Goods Vehicles Travelling to Mainland Europe”. A copy of this bulletin is available on the DfT website at:
http://www.dft.gov.uk/stellent/groups/dft_transstats/documents/page/dft_transstats_611744.hcsp.
Based on DfT's “Survey of Foreign Vehicle Activity in GB: 2003” it is estimated that foreign heavy goods vehicles account for about 3 per cent. of all heavy goods vehicle kilometres on GB roads.
Yes; all motor vehicles when in use in the UK, including heavy goods vehicles registered abroad, must have insurance cover for third party liabilities meeting the minimum levels of cover required by Section 145 of the Road Traffic Act 1988.