Think you are getting confused re: nominal value and actual value. Its best to completely ignore the fact that nominal values are denominated in currency for the purposes of a question like this. They might as well be denominated in roflcakes or lolcats. Nominal values are not really worth worrying about.
RE: share premium accounts, if 10,000 shares were bought for £10,000 all that money would go to the capital account. All the same restrictions that apply to the share premium account apply to this account too. The company could use the money after a capital reduction, so this is a very viable possibility. I also wonder whether the Co could buy-back some of Peppa's/George's shares and resell those to Suzy/Delphine. THis isn't really a mechanics question though, not worth getting too bogged down here.
RE: the question, think carefully about the different things that could be done.
In both cases, the question says "invest" but doesn't say "shares". The money could be lent to the company like a bank loan.
Pros for Suzy: Could ensure she has a place on the company's board by an appropriate warranty in the loan agreement. Cons: As a nominee director would still owe duties to the company which may conflict with her wishes as lender. Doesn't give such direct control over the company.
Pros for Delphine: Arms-length involvement, lenders get paid out before shareholders in the event of an insolvency. Could take security over the company's assets. Cons: No direct control over company. Frankly Delphine looks like she should be lending debt rather than buying equity.
Then I'd briefly consider the protections available to shareholders under the Companies Act. Whether Suzy/Delp have control by virtue of their shareholdings depends on how much they have, 50% to pass a OR and 75% to pass a SR, but note that Peppa/George will have a majority on the board (assuming they are both directors). But need to mention directors' duties and the protection of s260/s994.
Another option is to put something in the articles.
Suzy could use this to say she must be a director, Delphine could use it e.g. to limit the circumstances under Peppa/George can distribute capital and provide for appropriate safeguards. Peppa/George could insert an objects clause saying that the company's object is to produce rainwear/associated equipment (though there are problems with this).
Cons are that, although the articles are a statutory contract under s33, they are with the company and are thus difficult to enforce directly against the other shareholders. Obviously, they can be changed by special resolution, of concern if Peppa/George together (or just Suzy/Delphine) have 75% or more of the shares.
Another option is to create different classes of shares, there are various pros as you pointed out (e.g. Delph could ensure that she gets paid out in preference to the other shareholders on a winding-up), but of course the articles can be changed if a special resolution is passed, and in any event shareholders rank below creditors on a winding-up.
Another option is a shareholders' agreement.
Pros are that it is enforceable against the other parties to that agreement as a contract. It would be relatively straightforward to for there to be a provision saying that the company should not be moved away from rainwear etc., and it would be easier to amend the contract (where all parties agree) should everyone involved want the company to diversify in future. In Delphine's case, various protections could be inserted. Can include enforceable mechanisms for dispute resolution (such as buy-outs). No obvious cons.
In real-life Suzy would sign a Shareholders' Agreement and Delphine would be a secured lender, I think...