I'm in Durham, UK.

I have a plot of about 13.5m by 5m, which I believe all needs digging over whilst adding compost/manure. I think carrots and other root veg don't like freshly composted/manured ground so I don't know if it's too late for them? This is only my second year of having a vegetable patch so I don't have much experience yet. The soil is clay (topsoil) to about a spades depth with a stiffer clay below (glacial till I believe if anyone's interested). It's been raining for what feels like months here so the ground is fairly wet, which shouldn't be a good time for digging as far as I know, but we are expecting a frost next weekend, so I don't know whether to just do my digging tomorrow (or at least what I can; weather is meant to be rain in the morning but not in the afternoon) to take advantage of the frost on the newly exposed weeds or wait. The rest of January is supposed to be wet however. We've had a very mild wet winter here. Having said that, I was walking on the soil today and wasn't sinking into it, even with a full wheelbarrow.

I will have some Rhubarb crowns which I believe need to go in in January or February, but after that the earliest sowing will probably be March, possibly April if it's still cold here. I'm planning on double digging where the rhubarb will go.

Also, I've weeded the current surface as a lot of it which I didn't get round to last year was overgrown, although I haven't got rid of everything. Does it matter? It's probably about 70-90% clear of weeds on average, but then if I'm digging over everywhere does it matter if it's not 100% clear before digging? I was considering putting a sheet or something down after digging to keep the weeds down until sowing; are there any issues with this?

The book I was looking through either said to dig in the compost/manure in autumn, or when planting/sowing. Where it says when planting/sowing, does it mean the week before/around the same time? I'm a bit late for autumn, but I presume most plants wouldn't mind, except for possibly the aforementioned carrots. It will be 2 or 3 months before they get sowed though.

Sorry for all the questions; I've tried to research this but this is getting into the realms of experience and specific timing, which the articles and books don't always cover!