Thanks for your replies so far.
I just figured that it's possible to search for N-numbers by name - 109 aircraft are currently registered through International Air Services:
Click
Southern Aircraft has however more than 500 aircraft registered:
Click
International Air Services Trustee's website is a pain. But then again initially £100 plus £245 is the lowest price I ever heard of for this kind of service. Currently they provide a even lower price for the first year: £99
Click
Southern Aircraft charges £450 per year, incl. VAT (International Air doesn't even mention if their price includes VAT).
In relation to the total costs of owning an aircraft, the difference between these two prices is almost negligible. On the other site - why wasting money?
I think I need to do some further investigation on this issue...
Even if we will not buy this specific aircraft, we are probably going to buy some other N-registered aircraft within the next year, so that this question would come up again anyway.