http://www.bidet-shower.co.uk/_resources/cms/WRAS_eWRAStracts_from_the_UK_Water_Regulation_Guide.pdf
Please take a good look at the above link, if you don't understand what it all means then you need to find a plumber who has done the water regs course who does. As with most things there is always a way to install these fittings correctly. The trouble is you will, 100% guaranteed, not be able or willing, due to the very high costs & the space required to install these fittings correctly so that they comply with the Law, which is the Water Supply (Water Fittings) Regulations 1999. Should you go ahead & install such a fitting you of course will liable for a large fine if you are caught but as important you will be putting at real risk the safety of the water supplies not only in your house but for streets around it.
Please DO NOT install such fitting.
It really annoys me that these fittings are being sold when they know the score with the Law, course there is nothing to stop them selling fittings like these.
Sorry but no self respecting plumber is going to help you break the law Kovac, have you not got room to have a proper bidet with a tap that has an air gap, installed ?
I am not so sure they could find one to match the W.C pan. LOL