Sound advice (above) about checking with your planning department. If they say anything other than "It is OK, go ahead, you do not need to let us know," ask them why clauses (2)(a)(i) and (2)(a)(ii) in Section 55 of Part III of the Town and Country Planning Act 1990 do not apply. Part III is titled "CONTROL OVER DEVELOPMENT" and starts with "Meaning of development". Section 55, Para (2) says
"2) The following operations or uses of land shall not be taken for the purposes of this Act to involve development of the land—
(a) the carrying out for the maintenance, improvement or other alteration of any building of works which—
(i) affect only the interior of the building, or
(ii) do not materially affect the external appearance of the building"
Section 57 titled "Planning permission required for development" says in paragraph (1) "Subject to the following provisions of this section, planning permission is required for the carrying out of any development of land." Have a look at the restrictive provisions that follow (it is probably only paragraph (3) that might have any bearing on your situation) and if none of them apply your proposals for internal changes are not "development" and so planning permission is not required. I would be interested in their response. Best of luck.