There are a lot of plants that enjoy a heavy clay soil. But drainage is a good idea. Also, try to put in hard paths as the grass will quickly turn to mud if walked on too often.

Although drainage seems a major job, it's actually quite simple, if a bit of hard digging. All it needs is a channel or two cut out to about a foot deep, which is where most plants' roots bottom out. Fill that with shingle (20mm plus), cover that with membrane (to stop soil from clogging it and rendering it ineffective), then backfill. The channels should run from high point to low point, across the angle, so that water flowing at clay level drops into the channel and is carried away. Just make sure that the water isn't funnelled into your own back door! Perhaps angle the channels to a path edge, or the (lower) neighbour's garden (which is acceptable), or your (new) pond. Phil

Having established the drainage, I suggest roses, roses, roses all the way!