The UK market for Traveller's cheques has been in steady decline for a few years
, despite the overall increase in travel to Britain. (source Mintel Market Research - Holiday money UK). This has been caused by a number of factors, not least being the increased issuance of credit cards and debit cards, and the extent to which they are accepted abroad. Support for the use of cards and the undermining to Travellers Cheques has been further eroded by the massive growth in the worldwide ATM network. This is coupled with the perception (and fact) that TCs can be a pain to cash. Especially at weekends or if you run foul of local bank holidays (quite likely in Spain where there are local bank holidays!)
I can cite my own example of this. trying to pay by credit card in an Ibiza restaurant in 1988 was an offline transaction that had to be supported by a passport and they were most reluctant despite displaying the Visa symbol. Fast forward 20 years and you have no problems using cards in restaurants and bars, even in out-of-the-way places. The problem is more likely that your card issuer will 'stop' the transaction as suspect because it is not chip and pin. It is now routine to pay in Supermakets and virtually all shops by card; this wasnt the case a few years ago when cash was still king.
Cash and Travellers cheques do have one big advantage for the budget conscious its much easier to keep track of what you're spending especially if trying to keep to a daily budget!