EASA has nothing whatever to do with 'permit' aircraft. All aspects (certification, licensing, etc.) of any aircraft that is described in Annex II to Regulation 216/2008 is regulated solely by the national authority.
Why should a pilot's ability to hold an EASA Class 2 medical certificate have any relevance to his/her competence to fly an aircraft safely in IMC as opposed to VMC?
The holder of a UK PPL(A) can also hold an IMC, IR and/or FI rating, although they will be valid only on Annex II aircraft after 8 April 2018.
There are a few Annex II aircraft certified for flight under IFR - DC3 and DC6 spring most immediately to mind. There is life beyond the Cessna 150, you know.
To return to the original question: Yes, it is possible to get a UK PPL(A)issued with a self declaration (see
CAP1441). There is currently no form for initial issue but
SRG 1190 comes close.
SRG 1210 for the medical declaration