I am also on Wealden clay

But I have about 12" of good topsoil over it. Its never good to have the subsoil brought up, especially ours, I often think we should go into business making pots out of it! If possible I would pick out the clumps of subsoil (just any massive bits). My priority would be to add more organic matter. I would use well rotted horse muck, (too new and it will scorch seed/young soft plants) and anything else I can get my hands on - including possibly buying some topsoil or compost if necessary. I spread all my compost in about March/April ready for planting April/May. I would definitely start your own compost bins so you can use your own stuff going forward. However, you may also find a bumper crop of weeds this year as they get exposed to the light after turning the soil, so another option is to lay black membrane over the patch and just plant through slits. If you have bad clay, then you probably should incorporate some grit as well to open out the drainage, but the downside is you need a lot and then you have slightly stony soil. A cheats way is just to add grit in the planting holes/drills, but its not as good a solution.
This year I would probably add some pelleted chicken manure nearer planting time as well.
Your FIL has recommended spuds as they 'break up the soil'. Again it depends how bad the soil/drainage it is with you, but personally I've never found the need to do that. Rightly or wrongly, if you want to plant different things, personally I would go for it. Life is too short. I'd plant whatever you like, no point in growing something unless you like eating it!
Rows are usually recommended but a few veg like to be block planted as they are wind pollinated - sweetcorn is the most obvious one. In fact, I tend to plant short rows of several things like beetroot and lettuce because it suits my beds, so you could almost call that block planting.