Composite Primary Key on partitioned tables, and Foreign Keys
ALL unique constraints involved in a partitioning scenario MUST have the partition column defined as part of the base index definition (subset of the index key) - (not merely an included column). In other words, you will not be able to create a unique constraint on just the ID column in this scenario.

Set up a partition function and scheme.

CREATE PARTITION FUNCTION [PF_dbo_Test] (DATE) AS RANGE RIGHT
FOR VALUES ('2018-01-01','2018-02-01','2081-03-01')
GO

CREATE PARTITION SCHEME [PS_dbo_Test] AS PARTITION [PF_dbo_test] 
TO ([PRIMARY],[PRIMARY],[PRIMARY],[PRIMARY])




Create a table with a PRIMARY KEY that references the partition scheme.

IF OBJECT_ID('[dbo].[Test]') IS NOT NULL
    DROP TABLE [dbo].[Test]
GO

CREATE TABLE [dbo].[Test] (
    [TransactionDate] DATE
    ,[ID] INT CONSTRAINT [PK_Test] PRIMARY KEY CLUSTERED ([TransactionDate] ASC) 
    ) ON PS_dbo_Test (TransactionDate)




Now, try to create a unique constraint on the partitioned table without specifying the partition column.

CREATE UNIQUE NONCLUSTERED INDEX [UX_Test] ON [dbo].[Test] ([ID] ASC)
    WITH (
            PAD_INDEX = OFF
            ,STATISTICS_NORECOMPUTE = OFF
            ,SORT_IN_TEMPDB = OFF
            ,IGNORE_DUP_KEY = OFF
            ,DROP_EXISTING = OFF
            ,ONLINE = OFF
            ,ALLOW_ROW_LOCKS = ON
            ,ALLOW_PAGE_LOCKS = ON
            )





  Msg 1908, Level 16, State 1, Line 17 Column 'TransactionDate' is
  partitioning column of the index 'UX_Test'. Partition columns for a
  unique index must be a subset of the index key.




So, when you ask the question:


  I cannot decide whether to use composite FKs, referencing both columns
  of the PK, or single column FKs, referencing the id column supported
  by a unique index.


I'm afraid SQL Server has decided for you. You will have to define your FK's with both TransactionDate AND ID.