A three month notice period is very common in the UK, especially in senior positions. Any company that is not willing to wait for a three month notice period when hiring for a permanent employee is going to greatly reduce the range of good candidates available to them. If they need someone immediately, they will hire a contractor for six months while they recruit for the permanent position.
In addition, the recruitment process could easily take a month or two. For example:
- Day 1. Send in CV
- Day 5. HR department reviews CV, passes the best on to the recruiting manager
- Day 12. Recruiting manager reviews CVs, asks HR to arrange interviews
- Day 14. HR manager calls you to arrange an interview, but you miss the call.
- Day 15. You call the HR manager back. Arrange an interview for the following week.
- Day 20. You attend first round interview
- Day 22-26. Hiring manager interviews other candidates
- Day 27. HR contacts you to arrange a second round interview. The senior hiring manager isn't available for over a week.
- Day 36. You attend second round interview. Hiring manager asks HR to prepare a contract for you.
- Day 40. You receive your offer letter to sign. The company want a couple of weeks to order IT equipment for you.
- Day 54. Start new job.
Many companies move quicker than this, but the above is typical. It can also take much longer. At any point before sending you the offer letter, the company could decide they don't want to hire you. Hopefully you've got other applications on the go at that point, but if not it's back to day 1. At that point your 1.5 to 2 months has run out and you are now unemployed. Many hiring managers are prejudiced against unemployed people, which could make it harder to find your new role.