poppins congratulations!
What maternity rights you have depends on how pregnant you are. Previous service won't count I'm afraid, but for statutory maternity pay you need to be employed by the same employer for 26 weeks by the time you are 25 weeks pg. See here about maternity leave, here about maternity pay and here about working while you are pregnant.
If you work out that you are not entitled to SMP, you will probably be entitled to maternity allowance, see here about that.
It may be that your employer offers some kind of enhanced occupational maternity scheme, where they pay extra, more than the statutory amount. These schemes normally require employees to have a certain length of service before they apply, but if there is any discretion operated where you work they may choose to recognise your previous service and make an exception in your case. If there is an enhanced scheme, it would be worth asking the question certainly.
In terms of when you should tell them, you don't need to tell them legally until you are 25 weeks pregnant, but most people obviously say earlier than that. Your employer is obliged to do a health and safety risk assessment for pregnant women, so if you feel there is any chance there might be things that should be looked at, you should give your employer the opportunity to do that. Also, you may feel extremely tired, or sick during the first trimester especially, so you might feel it would be a good idea to tell your immediate line manager at least.
On the other hand if you are feeling ok, lots of people wait until after the 12 week scan, at least to tell colleagues generally. Personally I'd probably tell immediate manager soon, and not anyone else for a while.