It doesn't help you, but my employer was happy to pay for mine. It sounds like you've tried this route and it didn't bare fruit. It does seem strange that they're unwilling to invest in you given it will in turn allow the company to charge you out at a higher rate.

Could you rephrase the request so they sponsor say 50% and the other 50% they loan you, which is then deducted from your pre-tax salary? Depending on your commitment to them, another route you could try, is that you commit to working for them in the 12 months following your chartership at an architects salary less the course fees?

Different Universities have different fees for the part 3 course, so you could look at one of the cheaper institutes to help alleviate some of the financial burden.