Permits to fly can only be issued now to aircraft not eligible for a CofA. That excludes most factory built aircraft, but includes homebuilts and some ex-military and vintage aircraft.
The PFA can approve the issue of permits subject to certain limits.
You can import a foreign homebuilt. It should ideally be of a type that already has gained approval. In any event a very thorough inspection will be required.
The PFA web site has much information on
http://www.pfanet.co.uk/engineering_...n_leaflets.asp You should certainly talk to them before proceeding with an import.
On the other hand, buying an existing G reg'd aircraft with a current permit is a relatively straightforward process.
The concession to maintain a permit aircraft yourself is actually not a lot different than for a CofA aircraft, except that the signatory and approval for the work is a PFA inspector rather than a licensed engineer, and you do not need 'tagged' parts so long as the inspector deems them airworthy.