Right;
First off all the comment about not needing ventilation in an older house is valid, as long as it remains an ‘older house’ it’ll be OK! However, as soon as you install DG windows & doors, block up those ‘draughty’ air vents, close off those ‘old chimneys’ (why would anyone want to do that!) & install a few extra modern gas, oil or solid fuel appliances to stop you having to wear the 10 or so sweaters in the winter that they used to wear in 1908, the house is not going to be that much different from ‘a modern Tupperware box’ as far as natural ventilation is concerned!
Modern fossil fuel burning appliances need a good air supply to burn efficiently otherwise there is a risk that everyone in the house could end up suffering from the effects of Carbon Monoxide poisoning (headaches & drowsiness), or even worse in the event of a problem! That’s why BC insists on retrospective ventilation on modernisation projects, it really is not something you should neglect!
Getting back to your problem; remember BC cant force you to do anything if it’s no worse than it was before so you don’t need trickle vents on all the windows. New builds have them but if you have an older house there is no obligation to fit them & its better if you don’t have any at all; everyone always leaves them closed anyway! All BC want is positive ventilation of fossil fuel burning appliances so it’s better to install an ‘open’ air brick in rooms where you have a gas, oil or coal burning appliance; this is compulsory on new or significantly modernised builds; air also has to be ducted to the appliance if it’s away from the vent & enclosed under a worktop or in a cupboard. Generally, if you’ve modernised any habitable room (including fitting DG), you will need to provide adequate ventilation – an opening window! If not, you need to fit a extractor fan; these are also required, regardless in the kitchen/bathroom/utility in addition to an opening window.
Re the lintel problem, you probably don’t have problem but UPVC widows do not have the load bearing strength of timber so I would definitely take advice. Why don’t you just ask BC to advise you what you need to do? So you might feel a bit silly but anyone that hasn’t made a mistake hasn’t done anything, that’s how you learn! You will eventually have to get them in anyway so why not make them earn their money! They can’t ‘fine’ you & won’t have you dragged away at dawn to be hung, drawn & quartered like Mel Gibson was in Braveheart! The worse they can do is make you put right what is wrong & that can’t be bad can it!