自動販売機

で買えるのが英国です。
空港にあります。
SIM CARDSの自販機。
簡単に見つかります。

店舗なら、//www.heathrowairport.com/
Heathrow Airport shops A-Z

SIM Local
Location: Terminal 1, Arrivals | View on map
Opening Hours: 06:00 – 22:00

3UK, Vodafone, Orange, T-mobile
通信業者によって長所短所があるので、
ご自分の端末および使い方にあうものを
決める。店や機械（自販機）は石を投げれば
あたるほどあります。

一例
//www.three.co.uk/
Three/Support./Store locator
Covent Garden
Strand
Distance from you 0.19 miles
448 The Strand, London (Strand), WC2R 0QU, UK
Telephone: 0207 2408523
Opening hours:
Monday: 09:00am- 7:00pm
Tuesday: 09:00am- 7:00pm
Wednesday: 09:00am- 7:00pm
Thursday: 09:00am- 7:00pm
Friday: 09:00am- 7:00pm
Saturday: 09:00am- 7:00pm
Sunday: 11:00am - 5:00pm

London China Town
Distance from you 0.34 miles
Ground Floor, Wingate House,, 93-107 Shaftsbury Avenue,
London (China Town), W1D 5DA, UK
Telephone: 020 7851 0817
Opening hours:
Monday: 9:00am - 8:00pm
Tuesday: 9:00am - 8:00pm
Wednesday: 9:00am - 8:00pm
Thursday: 9:00am - 8:00pm
Friday: 9:00am - 8:00pm
Saturday: 10:00am - 8:00pm

英国ではなんら苦労することなく、SIMカードが買えます。
何より英語が通じるし・・・。