I am puzzled about the large time span for a possible burial/cremation, especially when you seem to know that the person would almost certainly have been buried/cremated within a couple of miles of Isleworth, but which part? It makes me wonder: has the death has been found in the GRO index to narrow the time down? By then ages were given in the index.
Be that as it may.
My thoughts are (and perhaps you have already done this and eliminated possible duplicate names, in which case, apologies):
- use wild cards if possible to search the indexes and consider what letters might have been confused for others e.g. St can sometimes look like H; 'll' can be mistaken for 'tt' and vice versa; W can look like M; N could be mistaken for V and vice versa (definitely seen that one)
- check the probate index for possible entries corresponding to GRO deaths;
if you can establish when the person died, check the online newspapers around the right date;
- check the deceased's parents' burial places to see if they were buried there.
All Saints' church at Isleworth has a churchyard but it ceased to be used in the 19th century. The cemetery beyond the hospital was opened in 1879 to replace it. I would imagine the burial records are with London Borough of Hounslow, but sometimes burials were recorded by the officiating minister in their own church registers as well as cemetery records. The registers of All Saints' are held by Hounslow Library. They were water damaged during the war and can be difficult to read. There may even be restrictions on viewing now due to their condition. Isleworth cemetery is now closed for burials, but I can't find out when it closed. Closure of cemeteries in the time period you are possibly looking at needs to be considered and therefore where might the next choice be. Inevitably it might be further from Isleworth itself. New Brentford cemetery for example is in Hounslow. Hounslow cemetery is still open and that too should be considered.
Possibly Gunnersbury cemetery - managed by LB Kensington & Chelsea, opened 1929.
There is a guide to all the cemeteries of London but it may be more of a historical guide than where the records are kept.
South West Middlesex Crematorium (aka Hanworth) is the only crematorium in Hounslow. However, some residents might have opted for Mortlake, depends on date. Certainly more recently that was another option. Both Mortlake and SW Middx has an online Book of Remembrance - doesn't seem to be searchable by name and may be too late for your purposes.
Good luck.
Nell