For Pakistan, India and the Gulf, an eSIM bought before departure avoids the airport SIM-shop queue and the risk of an unregistered number being blocked after a few days, a known issue with some Pakistani networks for foreign-registered SIMs. Compare a dedicated travel eSIM provider such as Airalo or Holafly against your UK provider's roaming add-on. For trips longer than a week, a local eSIM is almost always the cheaper option, but check your phone supports eSIM before relying on it as your only option.
Set it up before you leave, not after you land. Install the eSIM at home on your own Wi-Fi, confirm it appears as a second line, and read how activation works, since some plans start the clock at install and others at first connection. Keep your UK SIM active but with data roaming off. Your WhatsApp number stays your UK number regardless of which SIM provides the data, which is exactly what you want for staying reachable to family on both ends of the trip.
If you're arranging travel for parents or older relatives, do the eSIM installation on their phone before departure and test it. An airport arrivals hall with no working connection is the worst possible place to troubleshoot a QR code. And note most travel eSIMs are data only, fine for WhatsApp calls, but if a trip genuinely needs a local phone number for local calls, that's the one case where the local SIM shop still wins.