Most of the big contractors are very reluctant to let IWAs out any more. I used to have my IWA but let it lapse - noone will sign off a SSoWPs pack without me having an entourage of protection staff.
Never heard of a IWA using ATWS for protection though it's theoretically possible. TOWS is great if you can get the key to it and it's still maintained.
Of course if you're NR maintenance different rules apply (not a dig, but in the contracting world Red-Zone is effectively banned).
I used to COSS and IWA for both contractors and Network Rail as staff, but everthing has tightened right up now.
Taking T-12's and T-2's used to be nice and easy!
Where IWA will benefit you is if you're working in a blockade, as I found it a lot easier to sign in with the ES to do my inspection, verification and survey works. Again, SSoWPs dependent now.