Extract multiple values from string

# Define the regex patterns to search for indidvidually, as elements of an array.
$patterns = 
    # A string literal; escape it, to be safe.
    [regex]::Escape(' in the case of a single acquisition the Total Purchase Price '),     
    # A regex that matches a currency amount in pounds.
    # (Literal ' £', followed by at least one ('+') non-whitespace char. ('\S')
    # - this could be made more stringent by matching digits and commas only.)
    ' £\S+',     
    # A string literal that *needs* escaping due to use of '(' and ')'
    # Note the use of a literal here-string (@'<newline>...<newline>'@)
    [regex]::Escape(@'
a copy of any financial due diligence
reports obtained by the Group in relation to the Acquisition Target, on a non-reliance
basis (subject to the Agent and any other relevant Reliance Party signing any required
hold harmless letter) and a copy of the acquisition agreement under which the
Acquisition Target is to be acquired;
'@)

# - Use Get-Content -Raw to read the file *as a whole*
# - Use Select-String -AllMatches to find *multiple* matches (per input string)
# - ($patterns -join '|') joins the individual regexes with an alternation (|)
#   so that matches of any one of them are returned.
Get-Content -Raw $SourceFile | Select-String -AllMatches -Pattern ($patterns -join '|') |
  ForEach-Object {
    # Loop over the matches, each of which contains the captured substring
    # in index [0], and collect them in an *array*, $capturedSubstrings
    # Note: You could use `Set-Variable` to create individual variables $Variable1, ...
    #       but it's usually easier to work with an array.
    $capturedSubstrings = foreach ($match in $_.Matches) { $match[0].Value }
    # Output the array elements in diagnostic form.
    $capturedSubstrings | % { "[$_]" }
  }

^{Note that -Pattern normally accepts an array of values, so using -Pattern $patterns should work (albeit with subtly different behavior), but as of PowerShell Core 6.1.0 doesn't due to a bug.}

Caveat: The assumption is that your script uses the same newline style as $SourceFile (CRLF vs. LF-only); more work is needed if the two differ, which would surface as the last pattern (the multi-line one) not matching.

With a file containing the contents of $String above, this yields:

[ in the case of a single acquisition the Total Purchase Price ]
[ £5,000,000]
[ £10,000,000]
[a copy of any financial due diligence
reports obtained by the Group in relation to the Acquisition Target, on a non-reliance
basis (subject to the Agent and any other relevant Reliance Party signing any required
hold harmless letter) and a copy of the acquisition agreement under which the
Acquisition Target is to be acquired;]