No, it's because your positioning flight is a separate booking - so if it arrives late or is cancelled and you miss the departure for your QR ticket in the 'positioning' airport, you have lost the entire QR trip (outward and return). Your travel insurance might pay up for a 'missed departure', but they also might not if they grasp all the circumstances.

Positioning overnight is normally the recommendation, unless you can very clearly identify that the "out to position" flight will be the same aircraft as the first leg of your onward flight back through the UK (doing the positioning literally "back-to-back"). Your proposed gap of 6 hours actually sounds generous enough that you might do a same-day positioning flight...but the key will really be what your back-up options are. You can probably cope with a delay of up to 4 hours quite comfortably, but what are your alternatives if the flight is simply cancelled...? If your ex-EU starts in say Amsterdam, you will have a lot of alternatives available i.e. other flights on BA you could move to, other airlines you could throw £100 at for a cheap fare etc. If your ex-EU is somewhere more niche like Bucharest or Sofia, your back-up options will be quite limited. That will determine how comfortable you really can be with this arrangement.