Ilford had a major problem with labs not liking XP1, it was a superb film but needed non a non normal standard process time in C41 developer something which few labs catered for. Ilford published times for their own XP1 developer ansd C41 which included push p[rocess times for speeds abovec 800 EI.
The film was reformulated as XP2 (later Super) and is designed to be processed at the standard C41 time. Ilford dropped the push processing recommendations because few labs would do it but in practice it's still an option and I continued to push process my XP-2 to 1600 & 3200 EI in C41 chemistry until about 2001/2. I remember discussing this with a senior Ilford chemist, and to sales managers, in the early to mid 80's and they confirmed taht while it was obviously still feasible the recommendations had been dropped because Ilford wanted a film with 100% processing compatibility with C41 minilabe etc.
For my use push processed XP1/2 gave far better results than HP5 etc.