A single text message via Vodafone from Spain/Portugal to the UK will cost you almost 60 pence. 40 pence goes to the foreign network provider, 10 pence goes to Vodafone, and then the dreaded VAT on top of your 50p.
If you make a voice call then it's possible that the line might drop for a fraction of a second and then reconnect. You don't know this has happened until you get your bill - try a 3 minute call when the line drops/reconnects half a dozen times and you're looking at about £3.00 on connection charges and then call time so your 3 min call costs £5.40 (plus VAT!)