Peers are to protest the laws which restrict protest within 1km of parliament. These laws are incredibly restrictive, and have caused the arrest of someone reading the names of war dead at the cenotaph, the arrest of someone for sitting on parliament square drinking tea and eating cake, and they prevent someone holding a placard outside parliament. Except for Brian Haw, who the law was designed to target, but is exempt from the law as his protest predates it.
The exclusion zone is not only problematic, but wide, maps are available here. One can be fined up to £1000 for being in the exclusion zone whilst a police officer thinks you're protesting - and the exclusion zone isn't marked at street level. Added to that the fact that people have been hassled in other parts of the UK, outside the exclusion zone, for wearing a T Shirt and it should be obvious that essentially a citizen can be arrested at will near parliament, as essentially anything could be an act of protest.
Good luck to the Lib-Dem peers, who are protesting having done the paperwork required by law.