Is your Extra Spend limit set to £0? If so that may be what is preventing you. The fact you are receiving texts means you are logged onto the roaming network OK.
You should be aware that Roaming calls from Antigua and BVI are in Roaming Zone 7. These attract charges of £1.70 / minute outgoing and £1.30 / min for receive. Data costs £5 a MB and texts are 40p each.
With data costs of £5 / MB, even if your spend limit is set higher than £0, you may have blown the spend limit without realising it if you left your data switched on by mistake. Hence why you cannot now incur more costs through new calls & texts.
As a recommendation, and I have travelled to Antigua before with a BT Mobile account, leave your roaming data switched off and only send texts when out of WiFi range (assuming you have left your spending limit high enough for the texts you need to send).
Then if you need to make phone calls, use the BT Smarttalk app to make calls using your home phone account via a WiFi connection (e.g. your hotels WiFi). Tell people to send you an SMS if they need to speak, then call them back via the WiFi connection when back at the hotel. We did this and it worked really well - calls are even free if you time them to be within any free call period in the UK.
As I say I strongly suspect that you may have spent money without realising it and have hit your spend limit, which would explain the situation you are experiencing.
I take it you haved tried to lift your spend limits by £5 to see if that brings outgoing texts back to life by logging into your account via a WiFi connection?
As you have found, if you are on BT then use of any roaming service other than SMS outside the EEA or the USA (where you can buy a roaming bundle) can be punitive.
The use of the 'Mobile Data Off' control before turning Airplane mode off when leaving the plane is highly recommended to avoid any chance of high charges (mobile phone background apps immediately start refreshing themselves and using data the moment they regain a mobile network). Also the selection of a hotel with good WiFi is pretty vital!
had to turn roaming on to send a quick watsap message... Which meant BT effectively charged me £40 for one text!
That's a bit disingenuous given that WhatsApp is not an SMS but a data service. I've no idea how much data messages use but if an image is included (what many use WhatsApp for, to avoid MMS charges) then 40 quids' worth of data at a fiver a MB could go in an instant.