I can’t see Apple raising the price of their 6.1” mid tier device by £100-200. They know people are now used to that screen size and it’s a dangerous game pricing their biggest market out.
Let's ignore the pound for a min because apart from China where Apple does special pricing to the iPhone 11, all iPhone prices ( Actually All Apple priced are priced the same ) are pretty much USD > Apple's exchange rate ( Which for Apple tends to be higher to hedge fluctuation in currency ) + Whatever Tax there is. ( Local Import and Sales Tax )
So nothing here suggest they will rise mid tier price by $200 or £200.
A gentle reminder here is all of my estimate / guess ( and Current estimate by many ) are based on "one" assumption. Apple will rise the price for next gen iPhone because of 5G.
Apple may very well have started "subsidising" iPhone RSP with their services margin already. At least iPhone 11 pricing in China suggest Apple isn't stringent with product pricing as they are used to be.
There is also another factor into iPhone Mid Tier pricing.
Apart from iPhone 7 and iPhone 8 generation where they rise the price by $10/$20 to $50 due to NAND and DRAM price hike. The Mid Tier Pricing of iPhone has been very consistent over the years.
iPhone $649 - $699, Plus Model is $100 dollar more expensive.
So if Apple were to reintroduce Two Model in the Mid Tier, it would make sense they follow the previous iPhone pricing. The $50 will be the difference between whether Apple choose to start with 128GB or not. i.e $649 they will retain 64GB baseline 5.4" , and you get 128GB for $699.
And all previous "new" iPhone Unlocked introductory price had always had a starting price of $649 even before the Plus Model appeared ( That is iPhone 6 ), if I remember correctly going back to 3GS. The last time we had a "new" iPhone introduce with $599 baseline was iPhone 3G, that was 2009. That is nearly $720 in today's dollar terms.
That is why I seriously doubt iPhone 12 5.4" will have a starting price of $599. Although I would love to be wrong.
I am also not sure where that iPhone SE Plus is coming from. I think it is likely Apple will stop updating the 5.5" Touch ID model.