SelfSovereignty is right about why this is happening. We pull our prices for each currency from the markets seperately so for GBP as in your example, we will pull and aggregate all GBP bitcoin prices just as we do with AUD. Given GBP and AUD have a much lower volume than USD it means their pricing is not updated as often, it has a small lag in price changes and the prices will be different depending on which currency you use.
As a fix to this, we will be updating shortly our system to reflect only BTC to USD pricing systems, from which we will convert the USD value into many different other currencies we wish to offer so that everyone shall receive the USD price (which makes up over 60% of the bitcoin volume as opposed to the GBP 4% and AUD 1%) which is converted to the currency of their choice using the lastest market rates. This should ensure any real price discrepancies are kept to below 0.5% and that each currency shall receive a fair weighting to purchase items and not discriminate against those using currencies forming only a small trade volume.
