The best and cheapest way is to buy a bare-root tree which are available from November to about March. However, you can buy potted trees which can be planted at any time but are expensive (I would expect them to be £100 upwards) and you want one supplied in at least a 40 litre pot. Here's an example website where such trees are available:
You need to rub them off (suggest wearing gloves) or use something like a nail brush to brush them off. Pesticides generally don't work on the adults so spraying isn't an option.
They are nematodes of some kind and are generally harmless or even beneficial. All soil contains nematodes but most are very small so go unnoticed.
Sow them when the seed pods are opening on the ones you are already growing. This gives them time to establish before they go dormant over winter. Spring sowing works but you usually have to wait a year before they flower.
1) looks like a carrot - maybe from birdseed. It could be one of a few other things but is definitely not a weed so would leave it.
2) is a poppy.
3) don't know but is another 'not a weed'!
I buy mine from Wickes. As long as it is 'sharp' or 'grit' sand and not 'builders' sand you should be OK. Sharp sand is gritty whereas builders sand has round grains. No issues with lime or salt with the stuff I buy.
It's a variegated weigela, possibly 'Florida Variegata'.
Here are some that aren't fussy and grow quickly: African daisy (Osteospurmum), Alyssum, Calendula, Candytuft, Coreopsis, Gypsophilia, Impatiens, Marigold, Mimulus, Nasturtium, Phlox.
Yes, I'd use annuals this year as you will get a display and they will outgrow grass. Perennials are often slow to establish which might let the grass take hold again. When the annuals have died off at the end of the year you can just keep hoeing the area over the winter (or cover it with bark etc) which will keep the grass and weeds down until it is time to plant your perennials. In general, the larger you can grow perennials in pots, the better they will survive when planted out so you could get some going this year and keep them in pots until next spring.
I find they much prefer being in the ground and don't really like growing in pots, especially in MPC. If you have to grow them in pots, I would put garden soil in them. I grow them here on clay which they love.