It is correct that the principal characteristic group (the characteristic group chosen for citation at the end of a name by means of a suffix or a class name, or implied by a trivial name) has a higher priority for numbering than any other substituents (expressed as prefixes). It is also correct that the compound that is given in the question does not have any principal characteristic group.
However, you are missing the priority for unsaturation (i.e. multiple bonds), which is between the priority of the principal characteristic group. The most important simplified criteria for the numbering of simple acyclic compounds are:
- lower locants for suffixes
- lower locants for multiple bonds
- lower locants for prefixes
- lower locants for substituents cited first as a prefix in the name
The corresponding actual wording in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
(c) principal characteristic groups and free valences (suffixes);
(i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings;
(ii) low locants are given first to multiple bonds as a set and then to double bonds (…);
(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
(g) lowest locants for the substituent cited first as a prefix in the name;
Since the compound that is given in the question does not have any principal characteristic group, the first criterion (greater number of suffixes) given in P-14.4 (c) is not relevant in this case.
According to (e), the correct name is (2E)-4-bromo-4-methylpent-2-ene and not (3E)-2-bromo-2-methylpent-3-ene since the locant ‘2’ for the double bond is lower than ‘3’.