Buying from PC World I can get base units for
£200 and complete systems from about £300. Why would I spend £100 extra to buy from you?
To be fair, there are sound reasons for not going to PC World to purchase hardware. One such example is the Motherboards the majority of the PCs have are not up to the job.
A big thing that companies fail to advertise is the "FSB" of a motherboard/hardware component. This is the speed at which the data travels along the computer's busses (Front-Side Bus).
So be careful

Even if a computer has 2.2GHz processor and a whopping 2GB RAM, if the FSB is only 633MHz, you're probably going to notice it.
Back onto the topic of his computer repair centre: is a retail unit appropriate for the major work you'll be doing? What percentage of your turnover is going to be directly associated with the Sale of Computers?
From my limited experience, the vast amount of hardware sales I've made are in the form of components - rather than whole systems.
Also, are people that bothered about seeing the computer in the shop? If not, why bother wasting £400 or whatever on a display model to sit and collect dust?
I strongly agree with your point about it taking up a lot of room. I live with my parents, and although they're very supportive of how well the business is now doing, I can tell that they're starting to get pissed off with laptops and computer desktops taking up an a entire room of the house.
Once I can truly justify the needs of a business property, I'll be moving too. But there will be two things I'll make certain of when I do:
1) The business pays for the property (e.g. rent) from its own bank account, and doesn't get into an overdraft/debt to a third party.
2) The business wont be moving to a retail outlet, as the majority of the sales are services related, so why spend a proportion of the rent in a retail unit when an industrial/office unit will be cheaper?
Is there no way you can carry on with the business at home, and perhaps pay your parents a bit of the profit as a way of keeping them sweet?
You don't want to have to give away a portion of your business unless you have no other option, but I believe you still have a few options available to you to keep it ticking over nicely.