Miss E - I was told by my employer and FAO that it is all taxed the same despite what you have previously earned. 15% is what it is, I am a high income earner and am being payed through employer and it is the same as it is through the FAO, that wouldn't be fair if it depended on how much you earned for the entire year, claiming it before July shouldn't make a difference, I got a big back pay of 9 weeks worth of it approx $5000 before tax and it was still only taxed at the low/weekly level. Briggsy's girl will know for sure but I worked full time up until I went on leave and in a high bracket and it hasn't made a difference in the amount of tax taken, my best friend had her baby a few weeks after mine and she is taxed the same as me despite earning less than half of what I did.
your employer is not taxing you correctly if he's not taking into account your annual figure. the whole point of ppl being paid by the employer is to make sure your annual income is taken into account! be aware that there is a chance you may end up with a tax bill at tax time due to the taxation rate your employer is using

there are no hard and fast rules as to which option is better - you really have to work that out yourself - playing with dates for it to be paid, different estimates etc. i worked out for our family bby is better because DH doesn't work - but if he worked, PPL would have potentially been better. no FAO staff will advise you to choose one or the other - simply because they will get a small snippet of the bigger picture. you need to work out what is going to be best using the estimator

i suggest trying several different scenarios and work out what is going to best work for you. if your income is really variable it may be safer to choose one option and stick with it - regardless of what the tax implications are or what you may look at later and say "we should have..." - you can only make the decisions based on the info you have at hand now