You are allowed claim a "reasonable" amount of your household expenses. 1/4 of a four bedroomed house seems excessive, as, presumably, the office, won't make up 1/4 of the square footage of the house.
Remember too, that you will not qualify for principal private residence relief on that proportion of the house you claim to be the office ie if you claim a 1/4 of the expenses now, 1/4 of a future gain on disposal would be subject to CGT.
I agree that 25% seems excessive, but I don't think there is any liability to CGT in future, unless you are actually charging your Ltd. Co. 'rent' for use of the fourth bedroom. If you are only claiming 'Office Heat & Light' expenses then as far as I know, there is no liability to CGT in future sale of the home.
I'm not an accountant so I'm open to correction on this?