Discrimination occurs when you are treated less favourably than another person because of your disability.
Cancer is automatically considered a disability, for the purposes of the Equality Act 2010, from the point of diagnosis.
This means that you are protected by the Equality Act 2010 and will be provided the rights contained within, such as the right to request reasonbale adjustments to your work and workplace. You do not have to jump through the loopholes of proving that your condition is compliant with the legal definition of a disability.
It may be that your employer is not aware that you would be protected by the Equality Act 2010.
Unforntunately, in these situations, assumptions can be made about the work that you can or cannot do, by your employer and colleagues. You do not have to inform your employer about a cancer diagnosis but being open may assist in ensuring that the correct reasonable adjustments are made, which are suitable for your individual situation. You can request that your employer does not inform anyone else about your diagnosis.
A study by the charity Mcmillan in 2016 found that nearly one-fifth of people (18%) diagnosed with cancer face discrimination from employers or colleagues on their return to work and 15% indicated that they returned to work before feeling ready.