Camisole | Tee | Button Down | Shirt Dress | Dress | Midi Skirt | Jeans | Cardigan | Jean Jacket | Moto Jacket | Rain Coat | Wellies | Sneakers | Flats | Scarf
From my experience, if you get about average weather, you can get buy with a pair of tights (to wear under skirts or dresses), jeans, and a light jacket (or two, to layer). You'll certainly want to pack your rain coat since it's unlikely you won't get any rain (or moisture) during your trip. And I'd recommend a pair of wellies--or walking shoes--if you're planning on taking walks through the countryside.
I think living in Colorado for 26 years prepared me well for England because dressing in layers is necessary, almost all year round. Choose pieces that you'll be able to layer--especially under your rain coat. During my first visit, two years ago this month, I got away with wearing only a canvas military-style jacket with a hood or a leather bomber jacket. On the coldest day, I just layered one under the other. I seem to live in a longer, thinner cardigan these days, and it's perfect for sunny days with a bit of a chill in the air, and easily goes under my rain coat. (Or my puffy coat which I'm sad to say I still haven't put away for the season yet.)
I recommend packing a pair of durable sneakers if you plan on doing a lot of exploring, especially by foot. I love Allbirds and can't imagine going to another travel shoe. I wore my yellow pair while we were in Berlin and have taken them to London a few times as well. My black pair served me well in Paris, even in the snow. They're made of wool and seem to regulate the temperature quite well because my feet stayed warm in snowy Paris and I haven't noticed them being overly hot on the warmer days, even with thin socks on. The best part is that after 15,000 or 20,000 steps, my feet don't hurt! When I get home, I just throw them in the wash and they're as good as new and ready for the next adventure. Seriously, get you a pair of these shoes! They come in so many colours now and they have men's and women's. I have two pairs of the Wool Runners and Luke just bought a pair of the Tree Runners.
The main thing is to stick true to your own style (obviously these picks reflect my own) and to wear what's comfortable and what you don't mind wearing all day, especially when it comes to foot wear. You may have your own philosophy on style versus comfort, but when travelling, prioritising comfort can literally save your trip. Trust me, I've been the Grumpy Gus who wanted to wear a cute outfit or cute shoes and was miserable from about lunchtime on.
And if you are planning a visit to the UK this spring don't miss these other posts:
A Muggle’s Guide to the Harry Potter Studio Tour
A Weekend Guide to the Cotswolds
10 Things to Do in Birmingham
15 Places to Visit in the UK
25 Things to Do in London
What to Pack for England in the Autumn
London in Bloom
Springtime in London
General Travel Tips:
5 Tips to Beat Jet Lag
How to Prep for a Long Trip
10 Items for a Day Trip to London
6 Mistakes I Made While Travelling Internationally