Keep it simple. If you don't understand, don't do it.
You seem to be a bit mixed up, wanting to invest in shares with a company with no shareholders. Legal & General have bought out Nationwide's unit trust business, but Nationwide are still an independent building society.
If you buy some financial magazines, you will see which are the largest unit trusts and which are performing well. Similarly with investment trusts.
I put my son's voucher into the "Foreign and Colonial Investment Trust Plc" if that helps. It's the oldest and largest investment trust.