Does anyone know anything about gardening?

I do, anytime from autumn to early spring when I get round to it. If you don't, on my soil at least, by midsummer the soil will be baked dry and cracking/crusty and the plants suffering from lack of water. Mulching makes a dramatic difference - if you lift up a layer of mulch on a hot day when it's not rained for ages, the soil underneath is usually still nice and moist. Also protects against cold damage over winter if you do it now. And if you don't, every year your soil will be slightly less nutritious than the year before (so, slower growth, fewer blooms etc) - with mulching it can improve slightly ever year instead.

Unlikely that most gardens would be able to make enough compost to do your whole garden, as per Monty Don's advice if I can't do the whole thing, I do some beds nice and thickly each year rather than trying to do it all but too thinly.

I've used bark - lasts well, looks nice, doesn't add much goodness to the soil; strulch - expensive, very easy to apply, didn't last very long, doesn't add much goodness to the soil, possibly helps reduce slugs; various bagged composts, soil improvers and mulches (essentially, compost with bigger chunks in) which have more or less varied in quality as you would expect according to their price. Homemade is better than all the above in my experience, possibly something about already being full of the right kind of fungi and bacteria etc for your garden because that's where it got made.

All of them have done a fantastic job at retaining moisture, make the garden look much better, and over time improve the structure of the soil. On my heavy clay I've found homemade garden compost and branded 'soil improver' type compost make the biggest difference to soil structure and help the plants grow best. They all suppress weed growth to some extent by keeping seeds away from the light - but will have the same effect on the self seeding of plants you like as well. In my garden the main weeds are perennials not annuals so this doesn't make much difference to me.

RHS says no less than 2 inches to have the desired effect, but ideally 3. Don't pile it up thickly directly touching tree or shrub stems/trunks as it could encourage rotting. Maybe try and avoid going too thick directly over the top of the crowns of perennials (depending on when you do this, you may not be able to tell). I've never found that bulbs have an issue - it's not like burying them 3 inches deeper in the soil because the mulch won't be anything like as compacted as soil. Most bulbs probably wouldn't care even if they were buried 3 inches deeper in the soil anyway - they 'swim' around a bit to find their preferred depth anyway. Possibly exception of tiny ones like fritillaries.