To make this process simple, download
SRG 1136 and fill it in. Go to your local flying club and do a SEP renewal with a UK Examiner, the school will provide you with a SRG 1119E Course Completion Certificate; the Examiner will provide you with a SRG 1119C on passing the SEP proficiency check. Include those forms with your 1136 and send to the UK CAA. You can get your medical issued by a UK AME; no need to involve the DGAC at all.