A few chemists in larger cities do stock Magnums in the UK.

However, mail order is probably the best way to go if you've already tried locally. This is the longest-running UK-based condom store online:

https://www.condoms.co.uk/acatalog/Trojan_Condoms.html

Bear in mind that if you purchase from outside the EU (as per the Condomania link above), single postal packages with a total value exceeding GBP18.00 will be taxable at a rate of
17.5% of (item value + shipping cost) + handling charge (between GBP4.00 for Royal Mail and GBP8.00 for Parcelforce, with Global Priority mail incurring a higher rate still).
In other words: buy from within the EU, or be sure to spend less than GBP18.00 when buying from outside the EU.