From what I've read, the house had belonged to the mans parents so would have been owned outright more than likely. For those who are not familiar with the area, Surrey is a very wealthy county with expensive house prices. These house prices have shot up dramaticaly in recent years. As an example, my grandparents bought their house for £9000 in the early 1970s. Today it is worth about £450,000 - quite an increase. Likewise my sister's house was bought in 1998 for £80,000 and has been sold for £280,000. With that in mind, this man probably owns his house outright so affording it probably isn't an issue.
In any case, a £28/hour on full time hours would wrk out as a salary of £54,600 before tax. After tax this would be about £38,000 so a very healthy amount of money if you don't have a mortgage or rent to pay. Also as he works freelance, he could be doing more than one job at once and could be paid different amounts for the jobs. I imagine that they are a rather wealthy family.
Regarding the family feud I just can't see it. They seem a respectable family. Feuds happen over money between family members but killing the adults of the family in a violent assault seems extreme. Also apparently the brother lives in Woking which is also in Surrey. Why would he have access to such violent weapons (I know it can happen) and would he risk taken them to France.
Although I have no evidence to suggest it, I still have a hunch that perhaps it could be related to the family ties with Iraq. This is just a hunch though.