Last time I went abroad I got Three PAYG after shopping around.
It looks like they’ve changed their offer since I last used it, but they offer free data roaming up to 20GB and Cape Verde is on their list of covered countries. Used to be ten quid for the month, dunno if it still is or not.
You (probably) don’t need a dongle unless you need your existing number to be reachable and don’t have a spare phone. Put the SIM in your phone, enable the Wi-Fi hotspot and then connect to that from your laptop as nickjb says.
Worth a look on moneysavingexpert, they’ll almost certainly have a round-up of the current best deals.