Don't go for the box/carpet bedding if you are pushed for time - box needs a lot of very time-consuming and careful maintenance, and carpet bedding is labour-intensive and very water-intensive too. I watch gardeners tending carpet bedding while I write in the university library, and they are there for hours and hours every week.
I would give the same advice to you as to someone starting an allotment: chunk it up, and put in a lot of time at the start - once you get a garden going it is much easier to maintain in the longer term. Being stuck in the preliminary stages, however, is like wading through treacle! If you can rope in some help from your DH, there's no reason why you couldn't easily be on top of the whole border inside a month, if you both put in a long morning each weekend.
Divide the bed into sections and work on one at a time. If the ground is currently empty in other areas, cover with weed-sheeting so you don't have to keep weeding the same area over and over again (this is exhausting and demoralising). It will look terrible for a bit, but better than than weeds.
Start by sorting out the problems you have with the soil - add absolutely LOADS of cheap compost (and I mean to the depth of 3-4 inches over the soil, which is bags and bags), and quite a few bags horticultural grit. If you want to get going straight away, you need to dig this all in. If not, you can leave it sitting there and let the worms do it for you. However, because spring is such a great planting season, and you're running out of time to get things like trees in before bud break, I'd get on with it and dig!
(I'm in Sheffield, btw, so I understand fully the problems of heavy clay and lots of rain!!)
Choose a colour scheme, and make a planting plan ensuring that you have something interesting every month of the year. Think about higher things (shrubs, trees) for the back then medium sized things, then lower things. You can get help here from magazines, which feature them for free, or even places like Crocus who sell border packs. Bear in mind that you need something that suits the aspect and soil that you have - there is absolutely no point putting sun-loving plants in deep shade, as they will die.