It's a national register of admissions to asylums. If they stayed in the asylum, and died there, the date of death is given. Making it very useful.
Yes, the Colney/Friern records are at the LMA. I've looked in them there (one patient and one attendant)
However, there are often surviving records at the Board of Guardians end, i.e. reception orders.
For example (completely random)
Louisa Marten
admitted Liverpool Road Workhouse (as Martin), 2 June 1863
Age 24, Single, "Lunacy"
Discharged 8 June, order of Magistrate, to Colney Hatch Asylum
Is in the Lunacy Patients Admission Registers, 1846-1912
Louisa Marten, admitted Colney H, 8 June 1863, "discharged" 6 Sep 1873 (died)
London, England, Poor Law and Board of Guardian Records, 1430-1930
(ironically, Islington did not at this time have a "Board of Guardians", like some other places it had been exempted from some of the provisions of the 1834 Act)
Islington / Register of Lunatics / Orders for reception and conveyance of pauper lunatics, 1863
Images 452-455 (of 681)
4th June 1863
Examined by Arthur Ballantine Esquire a Justice of the Peace
Louisa Marten, a Pauper Lunatic
Female, 24, Single
Domestic Servant, Church of England
of 2 Gerrard Street
First attack, duration about a fortnight, cause not known
Is suicidal, not dangerous to others
Chargeable to St Mary Islington
No known relatives
Samuel Richard ?, Member of the Royal College of Surgeons,examined Louisa Marten, 4 June
"Is very melancholy and desponding Feels her head is not right says her case(?) is going wrong that she is not answerable for her actions - They have exploded her friends and that has broken her heart
says she has attempted to drown herself twice is very sorry for it + fears she shall not be forgiven - Cannot council her thoughts"
"I hearby certify that I delivered the within named Lunatic with a duplicate of this Order at the Colney Hatch Lunatic Asylum at 1 p.m. this 8th June 1863"
(signed) George Moss
Certainly worth looking into
John