How would you rate the UK Open University's math BSc programme relative to prestigious ones?
The question is too specific about one particular programme at one particular university to be useful to other readers in this Q&amp;A format and is likely to be closed.
However, despite saying that, it touches on an area worthy of answer.

However: tl;dr Yes, it is considered equivalent to all other UK Honours degrees in Mathematics by other universities and would be acceptable for any postgraduate studies.

Now to the detail.
Some might consider it less worthy because it has no specific entry requirements. This is because it is designed to provide a route for a university education for those who may find themselves unable to attend a regular university programme. In particular the university was created to educate the late returned to higher education and permit them to traverse any glass ceiling or financial burden that previously might have held them back. This is a false premise. Students have to undertake rigorous studies and complete the same assessments at the same levels as students elsewhere, and in addition they often do this whilst still in employment or in a caring role. Many consider an OU achievement as more worthy because of this.
Some might consider it  less worthy because it is based on distance learning withe most of the delivery being done by the medium of the internet, videos and downloaded material. Very much like a MOOC.
This also is a false premise. The OU was very early in the development of remote online teaching and is considered the forerunner and exemplar of this mode of delivery which has been honed by them over the last 50 years. It delivers results and has developed techniques in educational technology that are the envy of other institutions. In this era of lockdowns and isolations many universities are having to rapidly catch up with what the OU has been doing for decades.

So I am tempted to say: What pre-conceptions did you have about their style of education that made you think their qualifications were in any way lesser in value?