I am not familiar with the Brittania or its wiring, but here are some thoughts which are based upon Clive Fosters wiring diagrams which are to be found on this site, specifically page 14. These diagrams are for the 603, but the Brittania may be similar, except I think that the dip switch will be on the column, not on the floor.
The first thing that I note from this diagram is that the dip beams are driven directly from the dip switch, which one would expect to be man enough to handle circa 9 Amps for the dipped beams but perhaps not the circa 20 Amps taken by the main beams. So a decision has been taken to drive the main beams via a relay, which is actuated by the dipswitch, via the coil windings W1 and W2
The second thing I note is that the power to the main beams is not taken via the Light Switch, which is also unlikely to be up to the task, but from a separate supply taken from 21 on the fuse box. This appears to be an unswitched and unfused supply derived directly from the battery applied via the relay contacts C1 and C2
The third thing I note is that the supply to the headlight flash switch is taken from the battery via the fuse 17 - 18 and thence to the main beams down stream of the relay. Clearly the headlamp flash switch is reckoned to be up to the task, perhaps because it's only used intermittently. Can I just emphasise the point that this means that the headlamp flash switch does not drive the relay.
So, what could possibly go wrong? The symptoms you describe indicate that the relay is latching on when the main beams are selected, via either the headlamp flash switch or the dipswitch. You can get a relay to latch on by connecting the output contact to the field winding i.e. W1 to C1. If this is done a momentary supply to W1&C1 will mean that the relay will come on and stay on after the momentary supply is disconnected, as power is now fed back through the relay. This can be useful if your trying to catch a momentary flicker from a brake fluid warning device, but not in this application. To unlatch the relay it is necessary to disconnect either C2 or W2, which you have achieved by unplugging the relay.
So I surmise that your car is suffering from a misconection somewhere in the wiring that is connected to W1 and C1, as although this fault could perhaps have arisen in the relay you have already checked that out. Whereabouts this might have occurred is more difficult to pinpoint but I'd suspect that it's occurred somewhere in or around the column dip-main-flash switch. What you don't want, presuming that the wiring colours are the same as for the 603, is any connection between the wires coloured Brown-Blue and the wires coloured Blue-White
Good Luck
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