Right to request (RTR) part time is either 75% or 50% and works 3 weeks on then one week off, or two weeks on and two weeks off.
Several years ago, it was determined to be illegal to discriminate between someone just wanting more time off for whatever reason, and someone wanting more time off for childcare or what have you. BA have indeed given right to request part time to people who have no childcare or other caring roles, and the reason for the request was given as “I’m working too hard”. Saying that, part time is considered on an order of application basis, and granted or rejected case by case. There is often a long wait of about a year from date of request to actually working part time, in order to fit in around the needs of the business. Requests are often rejected outright, and some requests for RTR part time are granted on a purely temporary basis.
At the moment, you can get back from a trip on the last day of your available days, and the industrial or EASA required days off are within your unpaid part time days off. (The cabin crew have to have their industrial/EASA days off contained within their available to work days for both part time and leave, whereas the pilots have never had that). Pilots do have wrap days attached to leave, which may have been designed to account for that, but they do not currently exist attached to part time days off). BALPA have apparently compared part time rosters and full time rosters and determined that typically, part time pilots are working harder than their adjusted full time equivalents. ie a 75%RTR pilot is doing more than 75% of a full time pilot’s workload. However recent legal findings elsewhere suggest this might need to change.
With all of the above in mind, a request for part time does not require seniority, and your request will not be rejected simply because you are junior. That being said, whether junior or senior, you cannot assume your request will be granted.