I would never discourage anyone from filing reports, but I wonder whether an AIRPROX will get anywhere.
At this stage, unless you managed to get a sight of the reg of the a/c it will be very difficult to trace to get his side of the story. As such, the only info available to the AIRPROX board will be your description. As such, the conclusion will be Category C: Risk unknown. Confliction in open FIR resolved by pilots concerned (or something like that).
With regard to your wider question, you need to bear in mind that whilst it may be legal to fly, it might not always be easy. 3km is not much vis, and it is nearly always worse when you are in the air than it looks from the ground. Take a radar service if you can, fly at non-common levels (i.e 2350 ft instead of 2500ft) and comply with the quadrantal rules above 3000 ft if you can, even if you are technically VFR and VMC. Anything you can to minimise the risk. This winter/spring high pressure that has affected us in the south over the past few weeks has produced some horrible but legal visbilities which are not apparent until you get up into the air. It's beem like flying in a bowl of milk.