Entry into USA after awaiting green card & leaving for health emergency

Since you left the US while your Adjustment of Status application (I-485) was pending, and you weren't granted Advance Parole prior to leaving nor did you have H1b/H4/L1/L2/K3/K4/V status prior to leaving, your I-485 was automatically abandoned, according to 8 CFR 245.2(a)(4)(ii)(A). You probably have no way of entering the US to file Adjustment of Status again (and you should not enter on most types of nonimmigrant visa with preconceived intent to apply for Adjustment of Status during that stay), so you can only immigrate by doing Consular Processing at a US consulate abroad for an immigrant visa.

Your I-130 petition will continue being processed, and will presumably be approved. Because the I-130 indicated that you will do Adjustment of Status instead of Consular Processing, when it's approved, USCIS will hold onto it instead of sending it to NVC for Consular Processing. Then you would have to file I-824 to have it sent to NVC. This costs several hundred dollars and takes a few months. Another possibility, which I'm not too sure of, is that, while the I-130 is still pending now, you can contact USCIS to try to change the selection on the I-130 from Adjustment of Status to Consular Processing in the embassy in London. If they can get this changed, then you wouldn't have to file I-824.

As for your baby, the easiest way to reunite in the meantime (since it will be a while before you can move back to the US again) is if your husband or someone else brings the baby to you in the UK. You could try to enter the US as a visitor if you intend to leave on time and not file for Adjustment of Status while there, but it will be hard to convince the officer to believe you since you filed Adjustment of Status last time, and your husband is a US citizen living in the US. (Also, if you entered on VWP last time, it's likely your VWP cannot be used again due to having stayed beyond the end of it. And if you entered on a B2 visa, that visa is probably void for the same reason. So you would likely have to try apply for a B2 visa if you wanted to try the visit route.)