Fit check and affordable recommendations for petite 28c/d in UK??

Hi been reading through this subreddit and was hoping some of you maybe able to help. I'm very new to bra sizing and Reddit so apologies if anything been done incorrectly.

Previously wore 32a (UK), band very loose and some slight gaping at top. Tried the abtf calculator and got suggested a starting size of 26d/dd, with the sister size of 28c/d.

26 band sizes seemed hard to get and expensive (on tight budget) so would prefer to get 28 and alter the band if needed. Also happy with second hand :)

Any recommendations for style, size and brand would be greatly appreciated!! Based in UK if that helps.

Have listed 6 measurements and bras I've recently tried on.

6 measurements (UK sizing, inches, AFAB) Loose Underbust: 26.5 Snug Underbust: 26 Tight Underbust: 25 Standing Bust: 29 - 30 (varies with cycle) Leaning Bust: 30.5 - 31 (varies with cycle) Lying Bust: 29

I'm also quite petite with narrow shoulders, 150cm and around 50kg if that's helpful at all. Not sure on shape but fairly wide set and very soft breast tissue

Trying on bras - all from M&S own brand clearance section so far. All with swoop and scoop :)

28d wired and unlined balconette(?) bra: - gore lies flat but digs into sternum, some pain when sitting. - wire digs into ribs a bit not sure if this is because I have quite prominent ribs, a fit issue or I'm just unused to wearing wired bras? - some wrinkling in fabric and slight gaping at top if slouched (was on clearance for 99p so may just be poor quality) - band seems ok on tightest hook.

28c non wired plunge bra - gore hovers about 1cm from chest. - gaping at very top of bra mostly due to petite frame and digs into armpits - cups too tall? - slight overspill near centre(?) If lean over or breathe in - Band rides up a bit at the back even on tightest hook - think the fabrics quite stretchy. - does give some cleavage that I don't think was possible though :)

30b strapless/multiway bra - overspill at top. - band too loose even on tightest hook (to be expected based on my chest measurements but this size is so much easier to find).

Thank you so much in advance and sorry for the onslaught of questions! Reading through all posts on here this feels like a very supportive community and very happy to have found it :)