Demergers are not defined in Tax Law but can be successfully used by Trading Companies and do get special tax treatment.
A demerger is a series of transactions which have the effect and purpose of dividing the trading activities carried on by a single company or group of companies between two or more companies or groups of companies. CTA10/S1075 and TCGA92/S192 provide special tax treatment if certain conditions are met. Companies may seek advance clearance under CTA10/S1091 that proposed transactions will be an exempt demerger. CTM17200 onwards gives further guidance on the action to be taken by local offices in dealing with demergers.
Basically there are 3 ways to do Demergers
Property Investment Companies are not trading companies so demergers are extremely complicated as explained in this article in Taxation