Jacques Hammer
Firstly, given a landlord has to give notice to enter the property, people shouldn't be caught out in that way. I'm sure you aren't advocating landlords themselves breaching the TA in this way are you? ;-)
Secondly, as a solicitor of all people, you should know that just because there is a breach of a TA, it doesn't necessarily follow that you can get possession of the property. Serve the s.8, and apply under ground 12 if you want, but again, that will take time, with no guarantee that the Judge will agree with you.
My guess is, that if it is a well behaved pet, and no damage has been done to the property, and it was a last resort, that you'd probably only have about 50-50 of a Judge actually agreeing with you, by the time it gets to Court, and possibly a contested hearing. Chances are one of you will have caved by then because of questions of costs, and come to a settlement, but either way, it gives the tenant a few months at the least, between moving in, and this resolving, to try and find somewhere else which will accept a pet.