I’ve had chronic costochondritis with correlating thoracic spine pain for the better half of a decade. I’ve managed to cope better with the variable pain from day to day, but I haven’t found any kind of cure that gets rid of it, either. I also have torsional upper cross syndrome on my right side, which is the side of my worst costosternal pain. It took years to develop, and takes even longer to try and undo. Targeted strength training for my upper & mid back, along with stretching the shortened muscles, has helped make slow progress on fixing my shoulder, which is noticeably rolled forward and triggers my costo pain.

I use the following devices at home to open up the chest area & prevent bad posture that exacerbates the costosternal pain:

https://www.amazon.com/dp/B089M49HLY/ref=cm_sw_r_cp_api_glt_fabc_QVPQE1YVQ6EJFZ3WPES5

https://www.amazon.com/dp/B08T959SZJ/ref=cm_sw_r_cp_api_glt_fabc_WXC4520GJAY6VS87C61V

https://www.amazon.com/dp/B099CPCBS3/ref=cm_sw_r_cp_api_glt_fabc_2SGG7KTS7KMT92XXQXTX

https://www.amazon.com/dp/B00NH0XG7E/ref=cm_sw_r_cp_api_glt_fabc_Q3TSJ48ZMBZ16KDMZSTQ?_encoding=UTF8&psc=1

I ordered this from Amazon, perhaps it is available in the UK as well?

https://www.amazon.com/gp/aw/d/B089M49HLY?psc=1&ref=ppx_pop_mob_b_asin_title

Otherwise, it may not be ideal, but I saw an Amazon reviewer of the back pod say they used a “sports cup” instead with a towel for padding. That may be something to try - I did and it was way to hard on my spine.