You can get ID 30mmx0.5mm thick in packs of 10 here:
https://www.cyclestore.co.uk/wheels_manufacturing_spacers_to_work_with_30_mm_bb_shells_i_d_30mm_0_5mm_width_pack_of_10-ID_69110
This place in the US does 1mm versions, but I imagine postage will bend you over a bit (assuming they post to UK…..
http://www.treefortbikes.com/product/333222394644/1218/Wheels-MFG-1mm-Spacers-for.html
Or you could get a steel one from Misumi. The do a dimension configurable hardened metal washer. I plugged some numbers in & for something 1mm thick with an ID of 30mm & OD of 35mm it comes out at Eur8.54, per piece.
I’m not sure Joe Public can buy from them direct though. We use them a lot through work.
The part number for the specifics I plugged in is: FWSSSH-D35-V30-T1
and here’s a link to the part:
https://uk.misumi-ec.com/vona2/detail/110302398410/?HissuCode=FWSSSH&PNSearch=FWSSSH&KWSearch=FWSSSH&searchFlow=results2type
You can configure it down the left hand side & then click the check price blue box on the right.