If this were a parking or moving traffic contravention case dealt with by the Environment and Traffic Adjudicators then you'd still have a fair shot on appeal based on the wording in the notice of rejection, this would be done by bringing a collateral challenge against the notice of rejection. However with the Road User Charging Adjudicators it would be a battle just to establish that a collateral challenge can be brought, and I suspect it would be rejected anyway, as the main flaw in the wording appears to have been fixed and the remaining issues are not so clear-cut.
If the penalty amount on the TFL website increases to £180 at any time prior to midnight on 21 April, then you have an open-and-shut winning appeal.
If the penalty charge amount does not go up prior to midnight on 21 April, then you might want to consider paying the discounted amount while you can.
Of course that leaves aside any issues of signage, as google street view is not up to date with the new ULEZ signs we cannot say whether the signage is adequate or not, and you obviously can't go back and check given that you live in Wales. You could appeal on the signage anyway, but it would be a gamble as we'd be going in blind.