I discussed this long and hard with several accountants as I run a one-man company. The upshot was that if I could guarantee that my CPL would generate income for the company, I could run the training cost through it and get at least partial tax and VAT relief. However if it transpired that there was no evidence of income generated by the training in the company's books (ie. no aviation-related income), the Inland Revenue and Customs and Excise would view the training as a benefit in kind and tax it.
Although there seem to be a fair few CPL(H)s out there instructing through their own company (which would be justification for ofsetting training in this way), I'm fixed wing and there are relatively less CPL(A)s doing this.
At first I thought I'd suck it and see, however I changed my mind because I figured that the last thing I'd want when I'm skint after I've got my licence is a fight with the Inland Revenue.
Also note that I already have a solvent, established company which has been profitably trading for a few years. Setting up a company just to dodge tax and VAT is illegal as it is a 'sham company'.