Short answer: They are viewed as both roughly equivalent. MEng (or MSci for other sciences) is a 4 year course. BSc + MSc is 2 course (3 + 1 years).
Longer answer/other useful info: I studied an MSci in Physics not CompSci so there might be some minor differences between the subjects, I don't think they are significant.
Although not directly asked I also feel its import to point out that MSc and MSci in the UK are different (MSc is one year, MSci 4years).
Generally in the UK a single 4 year course is becoming more popular. Of my physics course ~60% did the 4 year MSci, ~10% did the BSc and then an MSc somewhere else and the rest only did the BSc. Now these numbers are only an example and details will depend on course and institution (many lower ranked uni do not offer a combined course).
In terms of content both options are likely to be similar. Possibly the BSc and MSc tends to be slightly more research/project based. Although this is very course dependent. I recommend you research the course syllabus for each option.
The main factor to recommend the MEng is funding. For UK students student loans are generally only available for your first degree. This means subsequent 1 year masters must be funded independently/by a bank loan, which is often prohibitive for many people. As you are a Spanish student I am not sure what the situation is for you. Obviously this is something you may want to consider.
In favour of the BSc + MSc it offers more flexibility. You can chose a different institution or a more specialized course after your BSc. The flip side is you are not guaranteed a place anywhere. Additionally many jobs do not require a masters so it gives you the option to only complete the BSc (someone else might comment on how true this is in CompSci).