Why does a UK university need to know my sexual orientation and religion?

• The employing professor will not have access to this data.
• You can omit anything in this form.
• The data from this form goes to HR and is aggregated there so they can prove to auditing bodies that the staff distribution is not skewed (i.e. that they are not discriminating on basis of whatever)

No, this information will not be made available to the hiring committee and not filling out this information will not have any negative consequences.

The university uses the data to monitor discrimination - from time to time, it is checked whether the percentage of successful applicants from minorities is roughly the same as the percentage of the respective minorities among the applicants. If the ratio is very off and the number of hirings was sufficient to indicate a "trend for discrimination", the university will take measures to prevent this in the future. And for this, they need to collect the data.

It would be illegal for them to take this type of information into account when making hiring decisions - it would be 'direct discrimination':

It is against the law to discriminate against anyone because of:

• age
  
• being or becoming a transsexual person
  
• being married or in a civil partnership
  
• being pregnant or having a child
  
• disability
  
• race including colour, nationality, ethnic or national origin
  
• religion, belief or lack of religion/belief
  
• sex
  
• sexual orientation
  

These are called ‘protected characteristics’.

You’re protected from discrimination in these situations:

• at work
• in education
• as a consumer
• when using public services
• when buying or renting property
• as a member or guest of a private club or association

https://www.gov.uk/discrimination-your-rights/types-of-discrimination

As other people have already said, many employers collect this information (while keeping it secret from the people making the hiring decision) in order to ensure that their recruitment process doesn't contain systemic 'indirect discrimination', which is:

putting rules or arrangements in place that apply to everyone, but that put someone with a protected characteristic at an unfair disadvantage.

https://www.gov.uk/discrimination-your-rights/how-you-can-be-discriminated-against

If you leave that section of the form blank then it should not count against you in your job application (often the form will say this on it somewhere).

There is an exception in that 'positive discrimination' is (since a change in the law quite recently) allowed in certain, quite limited, circumstances:

Employing people with protected characteristics

You can choose a job candidate who has a protected characteristic over one who doesn’t if they’re as suitable for the job and you think that people with that characteristic:

• are underrepresented in the workforce, profession or industry
  
• suffer a disadvantage connected to that characteristic (eg people from a certain ethnic group are not often given jobs in your sector)

You can only do this if you’re trying to address the under-representation or disadvantage for that particular person. You must make decisions on a case by case basis and not because of a certain policy.

You can’t choose a candidate who isn’t as suitable for the job just because they have a protected characteristic.

Disabled people When recruiting you can treat a disabled person more favourably than a non-disabled person because of their disability.

https://www.gov.uk/employer-preventing-discrimination/recruitment

See also: The Equality Act 2010 and positive action - Commons Library Standard Note

If you find that an employer is breaking the law in respect of discrimination, then contact the Equality Advisory and Support Service (free).

Some examples of the types of issues we have advised on:

• An individual who was unhappy about the way that the younger clientele at work treated him and spoke to him because he was an older person.
• A Trans individual, who had transitioned from male to female, who worked for a security company and reapplied for a security pass only to discover that the process for renewing her pass had disclosed the fact she had undergone gender reassignment surgery.

http://www.equalityadvisoryservice.com/app/about