NHS England doesn't fund rectal spacers centrally. They only did in 2018 and 2019 at a handful of cancer centres as a trial. There was also the ICEMAN trial (which would have been funded by the manufacturer). Rectal spacers were more important in the past before radiotherapy got so accurate, but that's not to say there's not still a benefit today, just that the benefit is less, and not worth the NHS paying for spacers for everyone.
Many hospitals buy a small number from their own funds for special cases, such as patients with IBS, or on lifelong blood thinners, but it also depends on the type of radiotherapy. Just occasionally, someone without a special reason will get one because the stock has hit its Use-By date, and would otherwise be thrown out.