Just drive at whatever speed you feel safe at, regardless of the speed limit on that road. Even if you did ever need to drive at 30 on a 50/60 road, (though I know you didn't in this case), there is nothing wrong with that when the road conditions dictate slowing down to that speed.
Whoever is behind will have to handle it. Tailgaters who don't read the road conditions are found dead in collisions with tree trunks, walls, telegraph poles and lamp posts every day. Others are found high in branches after aquaplaning. Your ability to read the road, which you're already showing, is what will serve you. :)
This can be potholes (that were not there when the road was given 50/60 limit), puddles, fog, bad rain, ice, cyclists, cars coming the other way, high winds, fallen branches, bad road markings, oily surfaces, gravel, surface damage, animals likely to dart out (for example when driving through a forest or in the early morning on country lanes). There are so many reasons to go slower than NSL.
There is not a minimum speed limit but we need to be seen to make due progress given the limit and conditions.
Because you felt that driving at 50mph was already dangerous given all the potholes and the narrow lane (as well as having just passed your test), you were within your rights to go slower and whoever is behind would be irked but they also don't want a shunt, so focus on what is ahead, and what feels safe to you.
Sounds like you're doing really well, so don't let tailgaters get to you because even if you had been doing 60, there would always be someone urging you to go faster. Well done on passing and getting out there!