ONE SIM card of T mobile, Vodafone, O2, Virgin, Orange, 3, GiffGaff, Lyca, Lebara, Vectone,... Buy online SIM card (Full, micro, or Nano) and write in the cheackout area of paypal message.
YOU ARE BUYING ONLY ONE sim CARD OF ANY NETWORK.
Mention in paypal about what UK network SIM card and what size you want.
Posted to registered PayPal addresses.
Same day post it in second class recorded Royal mail UK