if you feel comfortable in the basic shape i.e. Slim bottom and looser top and it suits your shape then stick with it. If you gravitate towards trainers/converse, swap to ankle boots (ash jalouse style with a little heel, or Chelsea boot maybe). Or change to trainers if you go for boots. What sort of jacket do you wear, how about a longline boyfriend style cardigan, or a fitted collarless jacket. Or brogues or loafers?
Change the Breton for a slim crew neck merino jumper (£25 in Gap at the mo, or something like the J Crew Tippi style). With or without a shirt underneath with the coller and hem sticking out ( you can buy layering tops just with a collar and hem so you don't see the bumps of the buttons). Or a sweatshirt. Or a plain but textured jersey top (I'm looking at ones from Cos on my shelf).
Instead of denim jeans, how about coated jeans, or skinny cargo trousers, or p/leather jeans or leggings. Lengthen your looser top with a longline tight jersey layer under if you don't like the top of leggings.
Instead of trousers totally, a p/leather above knee skirt and ankle boots and opaques. I'm not hugely inspired by skirts generally though this winter as it's very 70s trend led and I don't think that's hugely wearable, well not for the weekend activities I do anyway.
Shops? in addition to the above try Cos and Jigsaw, and J Crew. In fact, if you can't stretch to J Crew, take a look at their site, their lookbooks are excellent and putting together outfit is you might not think of.