No. Just no.
the "8 million" includes the likes of Boris Johnson and Joanna Lumley. It includes all the children born at british forces bases overseas. It's a massively misleading figure batted about purely for racist purposes
Quite, I have come across various people who were not born in the UK but are British Citizens as most of the previous generation have all been born and lived in the UK. This include children of diplomats, children of british forces, children of people who lived abroad for a time for other reasons and have since returned to the UK.
Also, it should be pointed out that some of these people have been living in the UK for a significant period of time. For instance one person I worked with who was born overseas and returned to the UK in their 20's retired several years ago!
Whilst I can also give examples of young children who parents have recently returned to the UK, the ONS has the figures in 2011 as 7,505,000 (11.9% of UK population) compared with 4,896,600 in 2001 (8.3% of UK population) and 3,835,400 in 1991 (6.7% of UK population). Meaning that this group has been getting bigger faster than the UK population as a whole, however still not really a big problem. Especially as at the start of the 1960's there were about 2.5 million (5% of UK population).
Also it should be noted that getting on for 1 million of those currently from other countries come from predominantly white English speaking countries (such as; USA, Canada, New Zealand, Australia and to a lesser extent South Africa), making them probably very difficult to identify as non British unless the speak and their accent gives them away.
You will also find that there will be a number of international students within those figures who come to the UK and pay much more in tuition fees than their UK or EU counterparts (subsidising UK students) and pay for accommodation and food (contributing to the UK economy) before then often returning to their home countries to work unless they gain employment in the uK (further contributing to the UK economy).
Yes there are some people who loose out when there are large numbers of people come to the UK to work for lower pay that the UK workers would be willing to work for, however others then benefit due to lower costs when they purchase things. Sometimes, in the case of building works, people can benefit significantly.
It is very difficult to give a clear answer, as there will be some impact on the rail network due to non UK born, however would our network be noticeably less busy if only UK born people were aloud to use the network then probably, but mostly due to the lack of tourists. However, if it wasn't for non UK born people would the network be more or less expensive to run and/or more or less profitable; that is a whole other matter and almost impossible to know and even if an answer was possible to form I would suggest that it would probably be a little more likely to be positive (as I think that it would if you tried it with any other element of the UK you care to pick including the NHS and education).