National Equine Health Survey (NEHS) 2018

The National Equine Health Survey (NEHS) 2018 received information for over 13,000 horses, giving a snapshot of the health of UK horses for one week in May 2018.
38% of horses had one or more health problems recorded.
Of all the disease syndromes recorded, 29% were lameness including laminitis (23% in 2017), and 7.4% were metabolic diseases (8.1% in 2017).
Laminitis made up 5.4% of all individual disease syndromes recorded (6.5% in 2017), and PPID 5.3% (6.0% in 2017).  43% of laminitis cases were first episodes and 57% were repeat episodes (49% and 51% respectively in 2017). 

However, without knowing how many individual syndromes were reported, these figures are not particularly helpful.  All we know from the report is that 38% of 13,873 horses, so 5,272 horses, had 1 syndrome or more.
So a minimum of 2% of the population had laminitis when the survey ran for a week in May 2018 (calculated 38% of 13,873 horses = 5,272 x 5.4% = 285 / 13.873 = 2%).
Similarly a minimum of 2% had PPID, and 2.8% had metabolic diseases - but does metabolic disease include PPID, or does it mean EMS?  How many had EMS AND PPID, and how many had PPID without EMS?

67% of horses were recorded as having a body (fat) score of 3 with 17.8% recorded as being thin (score of 2 and under) and 15.5% recorded as overweight (greater than 3). [However, previous research has suggested that owners tend to under-estimate their horses' body condition scores].

According to this article from Kentucky Equine Staff
Can Body Shape Predict Metabolic Issues in Horses? KER August 2018
reporting on research by Fitzgerald et al. at Queensland University presented at the 2018 Australasian Equine Science Symposium:
Fitzgerald DM, Sillence MN, De Laat MA
Morphometric measurements for identifying equine metabolic syndrome.
Proc. Australasian Equine Science Symposium 2018 7:29
cresty neck score (CNS), but not body condition score (BCS), neck or girth circumference, was positively correlated to a post oral glucose test insulin result that suggested Equine Metabolic Syndrome.

However, 
Coleman MC, Belknap JK, Eades SC, Galantino-Homer HL, Hunt RJ, Geor RJ, McCue ME, McIlwraith CW, Moore RM, Peroni JF, Townsend HG, White NA, Cummings KJ, Ivanek-Miojevic R, Cohen ND
Case-control study of risk factors for pasture-and endocrinopathy-associated laminitis in North American horses
J Am Vet Med Assoc. 2018 Aug 15;253(4):470-478. doi: 10.2460/javma.253.4.470
looked at 199 horses diagnosed with endocrinopathic laminitis after grazing and found that "horses with an obese body condition (i.e. body condition score ≥ 7), generalized or regional adiposity (alone or in combination), preexisting endocrinopathy, or recent (within 30 days) glucocorticoid administration had increased odds of developing PEAL, compared with horses that did not have these findings."