The big pharmacies that you would know are Boots, Asda, superdrug. Asda is cheapest of the 3 but boots offer 10% off 2nd order blue boots points...so might work out cheaper /same as asda.
I'm not sure on other pharmacies but I have heard of Voy, pharmacy4u.
Cost wise, Asda works out about £186 a month with postage for first few months. Can't remember the next price up. Some of the unknown pharmacies are cheaper.
Each pen lasts 4 weeks, but there is enough for 5 doses in the first 2.5mg pen (not sure about 5mg pen) the 5th dose is hard to get out of the pen, but is usable.