In my SV650 AL7 owner's manual, there are two service schedules printed: one for the US, and one for 'Europe & Oceania countries.' The US schedule states oil changes every 3,750 miles and spark plug changes at 7,500 miles. The Europe & Oceania schedule states oil changes every 7,500 miles and plug
inspection at the same time; plug
changes are given as every 15,000 miles. And the last time I checked, the UK is still in Europe until the end of October
so even Suzuki themselves acknowledge the plugs are good for more than 7,500 miles.
As posted above, Robinsons Foundry is doing an offer on the MR8E-9 plugs at £6 per plug at the moment, although delivery time is a couple of weeks as they are waiting for new stock.
I also asked SparkPlugs.co.uk and they told me they can get them at £5.84+VAT each, plus £1.50 postage. As they are not currently listed on the website, you'd need to call them to make an order. But they can be bought a LOT cheaper than list price.