Ah, if you are still breastfeeding, you have more options to choose from
I understand that expressing may not be an option for you, what with the commute and your children needing you when you are home (I also have a 5 year old and a 9 month old baby so know where you are coming from on that one!)
Lots of women do struggle to express very much, even with the very latest thing in pumps. Babies are much more efficient at getting milk than any pump can be, and pumps don't give you that lovely rush of oxytocin that gets the milk flowing. It takes a lot longer to feed the pump than it does to feed the baby!
I think you have a good case for getting hypoallergenic formula prescribed. If you see the right doctor and explain that you are returning to work, they should be helpful.
However, as your baby will be nearly 12 months old, you might not need to use any formula at all if you are happy to continue breastfeeding outside of work hours, especially as your baby is a good eater. Many mums find that their babies go for long periods in the day without milk when they are in childcare, then make up for it by feeding lots and lots in the evenings. It really depends on how you feel about carrying on with breastfeeding once you are back at work.
What kind of childcare are you thinking about? I'm wondering if a sympathetic childminder or nanny might be more flexible than a busy nursery (and also more able to ensure that your baby is kept away from milk and eggs at snack time).
I'm a registered childminder, and started looking after a breastfed little boy the same age as yours (2 weeks before his first birthday). He was one that never needed formula (he has no allergies, his mum just didn't want to use it). She gave him a big feed at 5.30am before she left for work, and she arrived to collect him from me at 3.30pm. As soon as she walked in the door, I made her a cup of tea and she fed him while we chatted for half an hour or so. He kept on breastfeeding happily until he was 2. It worked really well for them, and he thrived on the balanced but highly restricted diet I feed to my own son (who has multiple allergies). I didn't mind the half hour breastfeeding in my home every day, especially as this mum is a friend as well as a client, but you would need to find a child carer who could see things from your point of view.
If you want to talk through your going back to work options, you could call the National Breastfeeding Helpline (0300 100 0212) - they are very used to this scenario! Obviously, your sons allergies are an extra worry for you, and if you want to talk through that side of things, there are loads of us on this board who have been through similar.