Yes you can take your pigs to the abattoir without sidegates, you can also drive there at greater than 30mph through the towns, with no insurance, taking on your mobile, and with your seatbelt off. You can do all these things, but they are not legal :-)
The question was what were the regulations, which was what I answered, not what can you get away with.
If you do not have sidegates, the State Veterinary Service can (can not will!) condemn your animal as not being transported legally. It is then offloaded, killed, blue die stained, and the carcass sent to the incinerator. The abattoir will bill you not only for the kill, but also the cost of disposal, and of course you end up with no meat We know of two cases where this has happened with cattle. It generally doesn't but you take a risk.