isn't it a legal requirement to have 600 feet cloudbase to be legal ?
I think you may be thinking of a myth which is circulated, and even mentioned in the IMC ground exam. This relates to "out-of-practice IR and IMC holders recommended minima".
Legally speaking, the IMC rating allows you to fly "Let-down and Approach Procedures to published Decision Height or Minimum Descent Height and to undertake missed approach procedures." which means for me at Biggin Hill, I could legally go down to 373ft QFE on the ILS. Whether or not that is a "wise" decision is up to interpretation, (pilot skill, plane you are flying, hours of experience in situation, currency, weather... All play a part). I am sure if you ask, most IMC rated GA pilots most will know their own limitations.
Do an ILS at Luton or a PAR at Northolt and then low level transit.
that is definitely an option! But it really depends on the conditions. Cloud-base at 500ft, I wouldn't attempt that low level transit - I would divert to Southend Airport and take the train back into London.
The reason I want a neutral answer is that if it isn't going to be of any practical use I don't really see the point of handing over any cash.
I disagree here - the training was invaluable to my flying. I have gone flying with bright sunshine at 8am, forecasts were for clouds at 2000ft around midday, when I crossed the Thames around 10.30, I was in Solid IMC at 1800ft and Biggin had a cloud base of 800ft QFE. Even without the ILS, I could have done a bad-weather / low level circuit and land safely. Without the rating, I would have had to turn back into the sunshine, and possibly land at Cambridge where the weather stayed like this for several days. I have also flown several times in IMC whilst navigating despite both departure and destination airports were in bright sunshine. It opens up your options, especially for longer flights in the UK.
Do the training, it taught me how to use all the instruments properly, and how to navigate in poor visibility. (invaluable).
Best.
alex (ps: sorry for the long post)