I had the challenge of purchasing 93 sim cards for a school trip where we didn't want the students (parents) to incur excessive charges for roaming data/calls.
Although the majority of retailers can only provide you with 2 sims per customer due to fair usage policy either their own or imposed by the network there are a number of marketplace sellers on Amazon that will provide you with more freely or if you send them an email.
On this latest Pay & Go Go Go tarrif the inclusive 75 minutes, 500 texts & 100mb of data for £10 top up each month are doubled after 3 months with additional benefits every 3 months although the maximum number of minutes won't increase over 150 minutes or 500 minutes if you top up with £20 a month.
You also get:
O2 Rewards: 10% of your top-ups back every 3 months;
O2 Tracks: O2 Tracks puts the latest hits from the official Top 40 on your phone.
O2 Priority Moments: Find exclusive offers & experiences nationwide.
Of the big 5 mobile companies the price plan is competitive with the cheapest voicemail, calls to landlines (after the first 3 minutes) & Picture (MMS).
Some local stores charge between £1 to £5 for the sim & force you to buy an airtime voucher at the same time, whilst you can order 2 free sims direct from o2's website, delivery is within 4-5 working days compared to 3 days maximum from the majority of sellers on the Amazon marketplace. Prepaymania, Phones2Post & Eternal Communications were the quickest suppliers from my experience with delivery within 1-2 days.
At 1p with no commitment until you sign up it's not a bad deal, however if you want a better cost effective option the a good deal I've just found that GifGaf's PAYG sim offers better value with unlimited text\web & either 250 or 500 minutes for £10 or £12 per month.