Sometimes trying to save money costs a lot more in the end. Your planned journey is a disaster waiting to happen. Yes, if all goes exactly to plan you'll be fine, but public transport in the UK just doesn't work that way. You should expect a snafu somewhere along the line and with your plans that will probably result in you missing your flight to Oslo and the ticket being cancelled. There are plenty of low-cost options staying at or near LGW so get there the night before, even if you've already bought you bus ticket to Victoria.