That is true - except in the winter in a scenario like
OVC005
Tops 5000ft (stratus)
sfc temp -3C
You can't fly at 400ft
and you won't want to fly 200nm OCAS i.e. at 2400ft / 3400ft / 4400ft because you will be in IMC and are more or less guaranteed to pick up structural ice over that distance.
If you have an IR, you can climb up to VMC, fly the 200nm at the lowest practical IFR/enroute level (FL090 plus; I would just go for FL140 because that is where the Eurocontrol routings get good and there is less risk of dropping out of CAS which causes London Control to instantly drop you back to London Info and then you are screwed again) and descend at the far end.
Notably also if the tops are 5000ft then you can prob99 climb to VMC
while OCAS (e.g. FL054 if on the south coast) and this means you are not going to get trapped in hazardous wx while waiting for the IFR clearance; you can zoom up to FL054 on your own and be in sunshine while waiting for the powers to be to get their act together.
BTW, regarding the postings which fairly forcefully claimed the the EIR is just an IR with a higher MDA ... AFAICS this is not true because of the ban on flying STARs. The upshot of this "little detail" is that if e.g. your enroute section was at FL100 then the EIR is an IR with an MDA of FL100. I suspect this is a c0ckup.