It is possible to form an N reg group but an N reg cannot be owned by a non US resident.
Therefore the way to have an N Reg group is to form a US based Trust company and have shares in the trust. The US Trust usually based in Delaware being a US resident owns the aircraft is what goes on the registration certificate.
The 5% relates to the minimum share of a G reg group allowed by the CAA.
The DfT needs to give permission for paid training to take place in an N reg and this will be limited by the number of shareholders, IIRC, no more than 4 shareholders. That would need checking out.
There are plenty of outfits offering to form and manage N reg trusts in the backs of the various pilot magazines.
You might also ask an N reg flyer who provides their trust services and whether they are any good and get a idea of the costs.
The whole is a murkey area. Here is a useful link.
Willow Air Flying Club - Southend Airport, Essex UK