The discrepancy in footwear sizes is due to the conversion from continental to UK sizes.
A shoe made abroad as a 46 falls between 10.5 and 11, so might be stamped as either!
A UK made shoe will of course be one or the other, and more expensive, so your firm is saving money at the expense of comfort!
I can wear a size 11 UK EE width, and therefore have to be careful buying shoes made abroad, and depending on the source, manage to get into a 46 or have to go to a 47. I also have a high instep, so where as a 46 trainer might fit, a 46 boot might not, because it wont accommodate my instep. Check this guide and recommend it to your firm!
http://www.dbshoes.co.uk/measure.html
It is important that you get it right, as long term use of an ill fitting BOOT can do lasting harm to your feet, and store up problems for the future.
A trainer is flexible enough for the laces to pull the shoe to a snug fit, but a boot will be a lot stiffer.
As to the H&S requirement, it is the company's responsibility if they insist they supply footwear/clothing, to provide footwear etc. that FITS for the reasons outlined above.
If you provide your own footwear, it MUST be of the appropriate type and standard, but you should reclaim the cost against your income tax - and this applies to ANY clothing or equipment which YOU provide, which is essential to the proper conduct of your job.
When I worked in a hotel, we were provided with a uniform jacket to wear, but I had to have two pairs of black dress trousers, five white shirts, and a dicky-bow tie and black leather shoes.
I was able to offset ALL of the above against tax, by filling in an expenses form.
With my encouragement, the chambermaids successfully claimed back money for tights which were ruined through being splashed with the chemical cleaning agents used, as these were not provided by the hotel.
http://www.dssafety.co.uk/ Check out this company for reasonable prices, or Google safety boots, and look for firms which offer a variety of fits.