This isn't bad policy. If IVF infertility treatment (i.e. test tube babies) has a very low success rate on obese women with fertility issues, it does not make sense to spend a small fortune on such cases. Basically the NHS is saying that this treatment isn't going to work for fat women so we shouldn't waste time and money on it. Especially since it is not a life-and-death situation. What I don't like is the use of BMI to set medical policiy because it is only a superficial measurement with a poor correlation to body fat.
Is the low success rate just because the women are obese or because they are both obese and infertile because they have PCOS and it's not being treated properly? Without going into the gritty details, a lot of doctors give advice to women with PCOS that make things worse because they don't understand the disorder, which involves ovaries and hormones.