"Does anyone know why abortion is so very common in the UK mainland"
Actually, it's not common at all by global standards. This is from the Guttmacher Institute (figures are taken from peer-reviewed journals and are referenced at the end of the article).
• The highest annual rate of abortion in 2010–2014 was in the Caribbean, estimated at 65 per 1,000 women of childbearing age, followed by South America, at 47. The lowest rates were in Northern America, at 17, and Western and Northern Europe—both at 18.
• Across regions, Eastern Europe experienced the largest decline in the abortion rate, from 88 in 1990–1994 to 42 in 2010–2014. Despite this decline, there is a persistent gap in rates between Eastern and Western Europe (42 vs. 18) likely reflecting lower use of effective, modern contraceptive methods in Eastern Europe.
• The overall abortion rate in Africa was 34 per 1,000 women in 2010–2014. Subregional rates ranged from 31 in Western Africa to 38 in Northern Africa. There has been little if any change in abortion rates in these subregions since 1990–1994.
• For Latin America, subregional abortion rates range from 33 in Central America to 47 in South America. Rates have increased slightly since 1990–1994, but not by statistically significant amounts.
• Abortion rates in Asia have also fallen since 1990–1994, although not significantly. Asia’s subregions all have rates close to the regional average of 36 per 1,000 women.
• Highly restrictive abortion laws are not associated with lower abortion rates. When countries are grouped according to the grounds under which the procedure is legal, the rate is 37 abortions per 1,000 women of childbearing age where it is prohibited altogether or allowed only to save a woman’s life, compared with 34 per 1,000 where it is available on request, a nonsignificant difference.
You want to reduce numbers of abortions - make contraception readily available. All that restricting abortion does is to make abortion unsafe; it does nothing to reduce the numbers of abortions.