It depends on the employer's policy. Generally, it's more generous but this varies. One very, very important thing to consider is the rule governing when an employer has to pay you maternity leave. Many employers, for example, have a 6 month probation period. If you fall pregnant w/i that time, they can terminate you for just about any reason. It may not be the most ethical thing, but as far as I know it's not illegal.
There's also some rule - I have to look it up in the book they gave me when I fell pregnant the first time - about your having had to have worked there for 26 weeks prior to six weeks before you due date for the employer to be compelled to pay your maternity leave. So it's not a very wise idea to get a job w/the express purpose of having paid maternity leave.
If your employer is not compelled to pay your maternity leave, you could be - depending on how long you've worked in the UK and built up NI contributions - be eligible for Statutory Maternity Pay (SMP), which is about £100/week.