I would not use Euro T/C if I was you, as the benefits are stacked highly in the favour of those businesses which sell and exchange them.
Euro Travellers Cheques are generally sold commission free in the UK, but at a rate of exchange which is a few cents lower than what you would find in overseas holiday resorts. So during the purchase transaction alone, you may lose around 3%. Then when you cash them overseas, you may be charged another 2% or 3%. If you bring any unused Euro cheques back home with you, the vendor will most probably cash them for you commission free, but at a buy-back rate which could be as much as 15% lower than the selling rate, so it really is a massive loss.
Sterling Travellers Cheques are sold in the UK usually with a fee of 1%. In Salou, holidaymakers can usually cash them commission free, taking advantage of the better exchange rates which are available outside the UK. If you bring any unused Sterling cheques back home with you, the vendor will most probably cash them for you commission free, at face value, so without the prospect of any additional loss.
You are quite right about being careful regarding some exchange companies out there because they may advertise an attractive deal but will in small print (and most often in Spanish) you will need to cash a huge sum in order to benift from the deal. Always ask for a net figure before signing.
Jodie - If you take American Express T/C you will not have any problems cashing them.