I observed a Swimbabes class, then also to a baby class at our local pool - a "Leisure Park" rather than just a lengths pool.
The baby classes at the pool seem pretty much the same, but far cheaper and you only pay for the classes you go to, when you go.
Class size is limited to 10 (you need to book in advance, but not pay), warm baby pool, v patient, experienced (and lovely) instructor, dunking, splashing, structured learning bit of lesson, swimming through semi-sunken hoops, dive sticks, floating balls, floats, playing bit, then opportunity to go and play in the big pool. DS and I have been going since he was 7 weeks old. He was 2 last week, and could swim a width at 20 months.
If you live anywhere near the Barnsley Metrodome, save some £ and go there!