The CAA issued an information Notice 2014/170 in October 2014 detailing the procedure. This IN has now disappeared and was replaced by
ORS 4 No 1146
Also, can a 172 etc still be flown with a LAPL or NPPL medical?
In simple that is the only way you can fly it with a National PPL as you cannot exercise PPL privileges on an EASA aircraft. The exemption allows you to fly the aircraft under the privileges of a LAPL which means no IMC flying and at least a LAPL Medical. If you hold an NPPL, then you can continue to exercise those privileges with either a NPPL or a LAPL medical.
Explanatory Note:
The purpose of this exemption is to exempt pilots from the requirement to hold a Part-FCL licence in order to fly a UK registered EASA aircraft within the privileges of the LAPL, SPL or BPL.
The 2012 ANO 2009 amendment requiring pilots to hold a Part-FCL licence when flying a UK registered EASA aircraft was itself amended in March 2015. This makes use of a derogation available until 8th April 2018 under Article 12 of EU Reg 1178/2011.