Up until 1989 twice yearly dipping was compulsory.....we did it if I remember correctly once after shearing and again in the autumn but I may be wrong as it was 21 years ago! Then until 1992 we had to dip yearly which did happen just after shearing making it really hard to handle wet slimy sheep!
The dips used were organophosphate dips which we all got covered with.....actually they are a pesticide derived from chemical warfare stuff

and proved very dangerous to the operative and the environment. The reason folks dipped was that sheep scab was a notifiable disease and MAFF the forerunners of DEFRA wanted to eradicate it.
In 1992ish it became non notifiable and into the 90s there were lots of cases of people suffering from a kind of ME which was linked with dip.
They then started looking at safer dips and came up with synthetic pyrethroid dips....safer to people but more devastating to the environment

So they banned them and rehabilitated OP dips but under extreeme precautions as detailed above by hexhammeasure!
SPs are still used though and are the sctive pesticide in Dysect, Crovect and Spot on......so these products must be used with great caution.
We all used to take our sheep to be dipped with the big flocks......I used to take several trailers full and collect other folks few sheep too. These were the days before paperwork you understand! We sheared like that too.
Nowdays people use pourons with the smaller flocks and some with bigger ones, if you have lots of sheep >400 for example, it is ofter quicker and cheaper to get the mobile dip in.
The history of dip is very interesting as lots of people associate dipping and pour ons with preventing fly strike when it is actually scab that everyone has been trying to eradicate. Coopers....if anyone remembers them......made their name in the 1850s patenting sheep dip made from sulphur and arsenic

Its since the 2nd WW that the more toxic....well synthetic.....as I expect loads of arsenic around the place was hardly safe.....and chemical warefare stuff has appeared.
With ivomectin came the possibility of scab treatment without dip......
All facinating stuff!