Can anyone explain to me how the new rules for landlords on Wear and Tear Replacement are meant to work in the following situation?

The property is 35 years old and the fixtures including the gas central boiler are original. The latest Gas Safety Inspection recommends that this boiler be replaced.

Presumably, the cost of replacing the boiler itself would be an allowable deduction from my rental income under these new rules BUT it would make sense to do this as part of a general makeover of the flat. E.g. move the boiler from its present position in a hall cupboard, where there are issues with ventilation, to an outside wall in the kitchen and put in a modern kitchen. Similarly with replacing the old bathroom units.

The cost of so doing would amount to several years rental income but would, in my opinion, be worthwhile in terms of improving its letting potential and also the resale value of the property.

Would these other costs be allowable and if so do I simply carry them forward as "losses" until they are written off?

re new boiler
These can be claimed for against you rental income.
I suggest any queries on Buy to let,expenses etc be looked at on GOV.uk
All information is on the government site.

A new boiler can be quite expensive can't it. I do wonder why we small landlords are hounded to extinction when the Panama lot reveal the true state of greed and fraud among those who would lead us.

The number of small landlords is at a record high. Meanwhile if I fit a new boiler at home I have to pay the full amount with no scope to offset the cost.

Even if the repairs are substantial, that does not of itself make them capital for tax purposes, provided the character of the asset remains unchanged. For example, if a fitted kitchen is refurbished the type of work carried out might include the stripping out and replacement of base units, wall units, sink etc., re-tiling, work top replacement, repairs to floor coverings and associated re-plastering and re-wiring. Provided the kitchen is replaced with a similar standard kitchen then this is a repair and the expenditure is allowable. If at the same time additional cabinets are fitted, increasing the storage space, or extra equipment is installed, then this element is a capital addition and not allowable (applying whatever apportionment basis is reasonable on the facts). But if the whole kitchen is substantially upgraded, for example if standard units are replaced by expensive customised items using high quality materials, the whole expenditure will be capital.