Essentially I'm planning to make a lasagne next week that's half-and-half made from scratch and jars.

I'm going to make the red sauce from scratch, so veggies, mince, chopped tomatoes, tomato puree and spices, but for the white lasagne sauce I bought a jar of dolmio and the pasta sheets are from morrisons

AFAIK, the standard procedure is to make the red sauce first and then layer it in a dish

Bottom layer red sauce, then a layer of pasta sheets, then a layer of the white sauce from the jar, then repeat the previous three layers, then finish off with loads of grated cheese (yum)

Then you stick it in the oven for 45 minutes at 200.

Now here's my issue. Due to time constraints I'm probably going to have to do all of the prep the night before (making the red sauce and layering it all) and then on the night just sticking the lasagne in the oven

Is it safe to leave an uncooked lasagne for 24 hours lying in the fridge?

well your meat will be already cooked.. so hell yeah!! If you have any doubts wait until just before cooking before pouring the jar of white sauce over.

I wouldn't assemble it so long beforehand because the ragu will cause the pasta layer to soften, but only a little, too early which may make it curl.

White sauce just needs to be the one layer on top and the cooking temperature is too high, I cook at 160 degrees for 40 minutes in a fan oven.

Disagree with that!

Should go (from bottom up):

Meat sauce
Pasta
Meat sauce
White sauce
Pasta
Meat sauce
White sauce
Pasta
White sauce
Cheese

I'd agree with that but lots of recipes differ slightly e.g. middle layers meat on top of white sauce and even meat and white sauce on top layer before cheese applied.
None though have the Bechamel only on the top layer.

As for the "safety" issue I dont see a problem but I'd just store the two sauces separately in the fridge and "build" it whilst oven pre- heated - a couple of minutes at most.
I once did what you propose with dried canneloni tubes - filled them and left them in fridge overnight. The tubes had split quite a lot. No big deal when cooked in sauce but I was surprised.

Didn't spot this post.
Agree with first paragraph.
Disagree with second.

Should go (from bottom up):

Meat sauce
Pasta
Meat sauce
White sauce
Cheese
Pasta
Meat sauce
White sauce
Cheese
Pasta
Meat Sauce
White sauce
Cheese + more cheese

What Degsy says, but no cheese.

No, lasagne should just be meat sauce, pasta and cheese. Use the white sauce to hang wallpaper with, it adds nothing to the dish.

My Onken Natural Yoghurt has been in the fridge since well before 3rd December 2015, when it "expired", and it has been open since some time in January. I'm still eating it, and it's fine. I doubt anything fresh will go bad in a fridge in 24 hours.

What you means is "I only like meat sauce, pasta and cheese".
Which is fine.
It's just not lasagne.