Ok here are a list of good vantage points the flight will be directly above or within a quarter of a mile ish to you

from BOREHAM Essex
Writtle
>>>>(HYLANDS PARK should be a good vantage point if you stand and face towards Chelmsford city center they pass just NW but not overhead)
Highwood
Blackmore
Stondon Massey
Kelvedon Hatch
Navestock Side
Hainault
Fairlop Waters
Barkingside
Wansted
Leyton
Hackney Marshes
Stratford
City of London
Buck House

Please remember you will need a good view around as the list above is going to be close but nearer London you get the lower they will go so a horizon will be needed low
Please do not blame me if you miss it I just used a pen and paer on a map form Buck House to Boreham Essex two known points.
1pm over the palace so in the middle of Essex get yourself settled by 12.30 latest

Im in