Yep NFL games at Wembley and Spurs are subject to the clear bag policy. It, or something similar, was policy at Manchester arena for a while too (no idea if it still is) - someone I was with had their bag taken off them and binned because it wasn't suitable.
Clear bags are really easy to find over here btw. All the online fashion retailers have them and Amazon have loads https://www.amazon.co.uk/Transparent-Toiletry-Waterproof-Crossbody-Adjustable/dp/B08N1CN6Y1/ref=sxin_13_ac_d_rm?ac_md=2-2-Y2xlYXIgdG90ZSBiYWc%3D-ac_d_rm_rm_rm&cv_ct_cx=Clear+Handbag&dchild=1&keywords=Clear+Handbag&pd_rd_i=B08N1CN6Y1&pd_rd_r=59878f77-f928-4f32-8871-a1bd6db4cda7&pd_rd_w=d1xhu&pd_rd_wg=1Mf8Q&pf_rd_p=73573abc-9548-43f0-87cb-a185286cee4c&pf_rd_r=7HJS8BZJX2BNPPD7YV9X&psc=1&qid=1628670218&sr=1-3-fe323411-17bb-433b-b2f8-c44f2e1370d4