On Monday the interbank rate for selling TL was £2.8192. So NatWest charged you 2.12%. I think that is pretty good - but then I never bring many lira back and hang on to the little I do, so I never use banks for this anyway. Perhaps 2% is more than other places?
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Whoops, sorry, I noted down the wrong number. On Monday the interbank rate averaged 2.8217. So that means the NatWest charge was about 1.5/1.7% depending on exactly the time you converted.