If I remember rightly, 400AAL is the minimum for ICAO omni directional departures before turning. If elev is 55' then 560' is stated, then 100' is added by Doncaster.
There are many UK airfields without SID's where you routinely depart IMC and at night.
Is it the case that in this case as long as you climb at the assumed ICAO min 3.3% (around 2deg) your terrain clearance is assured. Otherwise there would be a SID with climb restrictions on it or visual dep only?
Interesting subject...