You can contact them if you like, but typically all students are placed on the BEng (even if they apply for MEng) and their performance is monitored to see if they will be suitable for the MEng instead.
For my university, we need to obtain 65% average in our second year to move onto the MEng degree. So yes, it should be possible to upgrade the course part way through. I believe those who applied to the MEng would be up against similar restrictions regardless of what they actually applied for.
I won't be able to get the 65% average this year (my second year was awful) so I am going down the BEng + MSc route as the MSc typically has far lower entry requirements, but is still going to be quite challenging. The MEng is good as it goes on student finance and the MSc does not. Either way, options exist.